Abstract
purpose. To describe the psychometric characteristics of the Impact of Vision Impairment (IVI) Profile and evaluate its validity and reliability over time and between different forms of administration.
methods. The IVI is a 32-item questionnaire developed to measure the impact of vision impairment on restriction of participation in daily activities in five domains of functioning. Each item is rated on a six-level scale from “no difficulty” to “can’t do because of vision.” The IVI was administered by trained interviewers to 115 people with impaired vision (visual acuity less than 6/12 or visual field deficit) who attended the Royal Victorian Eye and Ear Hospital, a vision rehabilitation agency, or a self-help group for people with impaired vision. Data were also collected on demographic characteristics of participants, cause of vision impairment, and distance and near vision. General health status was assessed with the Short Form-12 (SF-12) of the Physical and Mental Health Summary Scales. A subset of participants completed the IVI twice, either 1 to 2 weeks apart or by different forms of administration (different interviewers or self).
results. Internal consistency of total and domain average IVI scores was high (α = 0.80–0.96) and sequential elimination of items did not affect consistency. Total and domain average IVI scores correlated moderately with both near and distance vision (r = 0.21–0.31) but did not correlate with physical or general health or comorbidity. Total and domain average IVI scores correlated most closely with global measures of restriction of participation (r = 0.44–0.82). Principal-components analysis confirmed that all IVI items contribute to one underlying theme and tended to confirm two of the five domains: emotional reaction to vision loss and mobility. The first three components explained 43%, 8%, and 6% of the variation in the data. Guttman split-half reliability coefficients between different forms of administration and over time ranged from 0.73 to 0.94 for domain and total IVI scores. Mean absolute difference for domain and total scores between administrations was less than 1 step for all domains and the total score.
conclusions. This study provides support that the IVI has sufficient internal and construct validity to measure the effect of vision impairment on restriction of participation in daily activities. The IVI demonstrates acceptable reliability over a short period and yields consistent results between interviewers. The IVI can also be self-administered with assurance that the results will be comparable to those that would have been obtained by a trained interviewer. Therefore, the psychometric characteristics of the IVI support its use in assessment of the vision rehabilitation needs of people with impaired vision. Its stability over time indicates that it has potential to evaluate outcomes of intervention.
Difficulty with daily activities due to impaired vision is commonly reported by older adults and often does not correlate well with clinical measures of vision or ability to perform specific tasks.
1 2 3 Numerous instruments have been developed to measure vision-related quality of life, both disease specific and generic.
4 5 6 7 8 9 10 11 12 13 14 15 16 Most contain measures that capture a combination of visual symptoms, visual physical function, and performance of and participation in daily activities.
In vision rehabilitation, an instrument to specifically address restriction of participation (handicap) is necessary to add to measures of impairment such as visual acuity and subjective or objective measures of activity limitation. Restriction of participation is the limitation on activities that a person needs or wants to do that is experienced as a result of impairment or disability.
17 Ability to perform activities is important in an individual’s life, to the extent that those activities are necessary or desirable to the individual. For example, loss of the ability to thread a needle is more important to a person who sews than to one who does not. From individual response to loss of particular abilities arises the need to assess vision-related rehabilitation needs of individuals from the individual’s point of view, in terms of the importance of participating in activities.
The Impact of Vision Impairment (IVI) Profile was developed specifically to assess vision rehabilitation needs in the context of limitation of participation resulting from impaired vision. It contains 32 items that query level of restriction of participation in common daily experiences. The instrument has been designed to cover a broad range of issues in five separate domains of functioning. Items contained in the IVI do not cover clinical issues such as disease or symptoms, nor does it contain physical or performance-based items. Rather, its development has been guided by the World Health Organization’s model of human functioning and disablement
17 with the goal of capturing a vision-specific profile of restriction of participation.
For the IVI to be useful in the assessment of the rehabilitation needs of people with impaired vision, it must be both valid and reliable. In other words, it must measure what it intends to measure and it must yield consistent results. The purpose of this study was to describe the psychometric characteristics of the IVI and evaluate its validity and reliability over time and between different forms of administration.
Methods
To sample a broad range of personal experiences with impaired vision, participants in the study were recruited from the Vision Australia Foundation (VAF) a low-vision service provider, the Royal Victorian Eye and Ear Hospital (RVEEH), and independent self-help groups for people with impaired vision. Eligibility requirements for inclusion in the study were the ability to converse in English, visual acuity less than 6/12 and/or impaired visual fields, and age 18 years or older. All participants were informed of the nature and purpose of the study and gave witnessed consent to participate. Participants were recruited between March 1, 2000, and January 31, 2001. The study received ethics approval from the Royal Victorian Eye and Ear Hospital in accordance with the Declaration of Helsinki for research involving human subjects.
Participants recruited from the RVEEH and VAF had clinical eye examinations that included measurement of visual acuity and subjective refraction as a routine part of their appointments at these agencies. Best corrected visual acuity, near visual acuity, and main cause of vision loss were abstracted from clinic files for these participants. Distance visual acuity and main cause of vision loss for members of self-help groups was by self-report and was confirmed using a directional-E screening test. Distance visual acuity was grouped into three categories: less than 6/12 to 6/18, less than 6/18 to 6/60, and less than 6/60, reflecting vision at which functional deficits have been noted,
18 19 20 21 22 according to the World Health Organization’s definition of low vision and severe vision impairment.
17 Those with visual field loss and visual acuity of 6/12 or better were categorized separately. All participants with visual field loss alone attended clinics at RVEEH, and visual fields were measured on an automated visual field analyzer (Humphrey Systems, Dublin, CA) using a standard program (24-2 Swedish interactive test algorithm [SITA]; Humphrey Systems). Visual field loss was noted when the results for the hemifield test (Humphrey Systems) were outside normal limits. Near vision was categorized as N8, N20, or N48 or less. Near vision was not available for members of self-help groups.
Participants answered demographic questions regarding age, duration of vision impairment, hearing loss, other health conditions, and limitations to performance of daily activities attributed to other health conditions. Participants also completed the standard 4-week Short Form-12 (SF-12, version 1; Physical and Mental Health Summary Scales)
23 and completed the IVI questionnaire. The SF-12 is a short version of the SF-36, a general health-related quality-of-life instrument. Two summary components related to the physical (physical summary scale; PCS12) and mental (mental summary scale; MCS12) domains of life were calculated from the questions in the SF-12, according to the algorithm developed by Ware et al.
23
All questionnaires were administered by interviewer to the person with impaired vision. All interviewers were trained to administer the IVI, using a questionnaire protocol developed for the study. Proxy answers were not solicited from caregivers or relatives to avoid biasing the IVI responses to the perception of another person’s opinion of the participant’s ability. A subset of participants completed the IVI on two occasions, to evaluate the reliability of the IVI between two interviewers and interviewer and self-administration and over a short period (1–2 weeks), before receiving vision support or rehabilitation services. Only participants with visual acuity sufficient to read large print (18-point font) were asked to self-administer questionnaires.
The IVI questionnaire was developed in three stages. Initially, focus groups to identify important activities that result in restrictions of participation were conducted with people who have the most common causes of impaired vision in Australia.
24 Issues explored included activities in mobility, household and personal care, consumer and social interactions, and leisure and work life domains. As a result of unstructured issue identification with these groups, a fifth domain was identified: emotional reaction to vision loss.
In the second stage of development, issues identified in focus groups were operationalized into a bank of 76 items. Existing instruments, including the Activities of Daily Vision Scale, the Visual Function (VF)14 questionnaire, the National Eye Institute-Visual Function Questionnaire (NEI-VFQ), and the Bristol Vision-Related Quality of Life (VQOL), were reviewed for content and scaling relevant to the issues identified in focus groups. The initial item bank was drawn mainly from the 139 questions in the VQOL and included 69 items with content that related specifically to restriction of participation and reflected the issues identified in focus groups. The content of many of the VQOL questions tends to overlap that of the other questionnaires reviewed, but question phrasing of the VQOL tends to be more closely aligned with the concept of restriction of participation. The VQOL contains a core of 10 items, and this core is included in the IVI. An additional seven questions were derived in areas in which the VQOL did not provide adequate coverage of issues identified in focus groups. Items regarding ocular symptoms and a person’s limitation in activities—for example, in seeing small objects—were not included in the IVI.
Trials of the IVI were then conducted in two consecutive versions of the questionnaire before deriving the final 32-item version. Trial version 1, containing the original 76 items, was administered to 96 people with impaired vision.
25 Trial version 2 was derived from trial version 1, with the number of items reduced to 45 by eliminating questions with a high degree of correlation (
r > 0.70) and items with floor and ceiling effects. Trial version 2 was administered to 256 people with impaired vision who had not participated in testing trial version 1. Data from this administration were then used to further reduce the number of items by eliminating similar questions with a high degree of correlation (
r > 0.60) and eigenvalue loading of less than 0.45 in the first component in principal-components analysis.
26 In both stages of item reduction through elimination of questions with high correlation, decisions regarding which items with similar content to discard were made in consultation with experienced vision rehabilitation professionals. All versions of the IVI have retained the core 10 questions of the VQOL and all have retained questions in each of the five provisional domains.
All versions of the IVI have used the question format of the VQOL.
16 Participants were asked how much their eyesight deficiency had interfered with an activity “in the past month.” Responses to items were rated from “not at all” (0), “rarely” (1), “a little” (2), “a fair amount” (3), and “a lot” (4) to “all the time” (5) and for some items, “can’t do because of eyesight” (5) or “don’t do because of other reasons” (8). Total and domain scores are an arithmetic average of the rating for applicable items. That is, when a participant rated an item 8, it was not included in the final average score. Items with similar content were provisionally grouped into domains in which specific rehabilitation interventions are directed. For example, in the mobility domain, intervention might include stair marking and orientation training, and in the household and personal care domain it might include provision of adaptive devices to aid in meal preparation.
All data were doubly entered in Access (Microsoft Access 97; Microsoft Corporation, Redmond, WA) and entry errors were confirmed by review of the participant file. Statistical analyses was performed by computer (SPSS version 10; SPSS Science, Chicago, IL), and
P < 0.05 was considered statistically significant. Only data from administration of the final 32-item version of the IVI are included in the analysis. In cases in which the participant completed more than one IVI for the reliability part of the study, the results from the first administration were used. (A trained interviewer always performed the first administration.) Bivariate Spearman correlation was used to determine the correlation between IVI item responses, item responses and the total IVI score, and each IVI domain and the total score with demographic characteristics. Analyses using visual acuity used the distance and near visual acuity groups at initial examination that have been defined previously. Analyses including near vision exclude members of self-help groups. Analysis of variance was used to determine difference between means of continuous variables for characteristics with more than two levels of categorization and
t-tests were used to determine difference in means between variables with two levels of categorization. The χ
2 analysis was used to evaluate differences in proportions. Internal consistency of the IVI and its domains was evaluated with Cronbach α, and Guttman split-half correlation was used to evaluate reliability between different forms of administration. The SE of measurement for reliability analyses was calculated according to the method of Fleiss,
27 based on the intraclass correlation coefficient.
Principal-components analysis was used to examine the underlying structure of the IVI. Sampling adequacy for principal-components analysis was evaluated with the Kaiser-Meyer-Olkin statistic greater than 0.5, and the Bartlett test of sphericity
P < 0.05 was used to determine that the population correlation matrix is not an identity. An initial principal-component extraction was used to determine the number of components needed to represent the data, by using eigenvalues greater than 1.0 or examination of the scree plot to define the number of components in the data. To simplify and interpret the components, a principal-components extraction specifying the number of components identified in the initial extraction was conducted using varimax rotation to transform the initial matrix. Items with loading less than 0.45 were considered unrelated to a factor.
28
Results
Study Participants
A total of 115 people, ranging in age from 25 to 98 years (mean, 72 years), participated in the study and a majority, 61%, (
n = 70) were women
(Table 1) . Forty-eight percent (
n = 55) were patients at the Royal Victorian Eye and Ear Hospital, 36% (
n = 42) were clients of the Vision Australia Foundation, and 16% (
n = 18) were members of self-help groups. Most participants were born in Australia 65% (
n = 75), 9% (
n = 10) were born in the United Kingdom, 17% (
n = 19) in Europe, 3% (
n = 4) in the Asia Pacific, and 6% (
n = 7) elsewhere.
The most common cause of vision loss was age-related maculopathy (39%,
n = 42). Glaucoma (21%,
n = 24) and diabetic retinopathy (15%,
n = 1) were the main causes of vision loss in similar proportions of participants
(Table 1) . Other causes of vision loss included six people with cataract. A total of 48% (
n = 54) of the sample had distance visual acuity less than 6/18, of which 39% (
n = 21) had less than 6/60 visual acuity. Four participants (
n = 3.5%) had less than 3/60 visual acuity. Fewer participants had better than 6/12 visual acuity with visual field defect (18%,
n = 21). Median duration of vision impairment was 4 years (range, <1 year to 74 years).
Most, 85% (n = 78) participants reported that they had an illness other than vision loss. Of these, common illnesses reported were arthritis, 18% (n = 14); heart condition, 29% (n = 23); high blood pressure, 19% (n = 15); asthma or other breathing difficulty, 29% (n = 23); and diabetes, 29% (n = 23). Many reported multiple other health conditions, 47% (n = 37). Of those, 32% (n = 25) said that it caused no difficulty in performing daily activities; 30% (n = 23), a moderate amount of difficulty; and 38% (n = 29) a lot of difficulty.
Validity
Twenty-eight (88%) of the 32 items were considered relevant by 90% or more of the participants (i.e., did not respond that they did not do the activity for reasons other than vision;
Table 2 ). Items 1 (work), 4 (sport), 5 (going out to events), and 18 (using transport) all had fewer than 80% of the participants respond that the item was relevant. The proportion that judged these four items to be relevant did not vary significantly by gender. However, for each item, those who judged these items not to be relevant were significantly older, on average (all
P < 0.001).
More than 20% of responses for all items were greater than 0 (no difficulty) and all but two items (20 and 21) had responses across the full 0-to-5 range. The mean score for the 32 items ranged from 0.92 to 3.58, and the SD of the mean for the 32 items was between 1.40 and 1.77 for 78% (
n = 25) of the items. The two items (20 and 21) that did not have responses in the most difficult rating had SDs less than 1.4, and all the items (1, 4, 5, 18) that were relevant for less than 80% of participants had SD of more than 1.77
(Table 2) .
Nine pairs of items had Spearman correlations greater than 0.60, of which three were 0.61 to 0.62 and one was greater than 0.70
(Table 3) . Three of the nine items were pairs of core items from the VQOL, and one pair contained an item from the VQOL core. All other Spearman correlations between all other possible pairs of items were less than 0.60 (data not shown).
Each domain and the total had high internal consistency, as demonstrated by a Cronbach α of 0.85 or greater for all but the household domain (0.80;
Table 4 ). Eliminating one item at a time did not change the Cronbach α for each domain or the total scores. Evaluation of the total score is limited by the four questions that had less than an 80% of participants responding that the items were relevant. The Cronbach α for the total score when these four items were eliminated from the analysis was 0.95 (
n = 86). The Spearman correlation between each item and the total mean score for the IVI ranged from 0.45 to 0.76.
The increase in IVI score correlated with decreasing categorical distance visual acuity for all domains and the total score (all
P < 0.05;
Table 5 ). Similarly, near visual acuity correlated positively with all but the leisure domain. Longer duration of vision loss correlated with decreasing IVI score for the social and emotional IVI domains and remained significant when data for participants who had outlying duration of vision loss (>70 years) were excluded from the analyses. Mean score for each domain and the total did not correlate with age or the general physical health component (PCS12) of the SF-12. Self-reported general health (ranging from excellent to poor) correlated with increasing mean score for only the leisure domain and IVI domain, and total scores correlated positively with the mental health component (MCS12) of the SF-12 (all
P < 0.05). There was no difference in mean score for any of the domains or for the total by self-report of other illness, nor was there a significant association between self-reported level of disability due to other illness and the IVI domains or total (data not shown). In multivariable models, only distance visual acuity remained significantly associated with IVI domain and total scores.
Two items in the IVI solicit global information on how vision affects restriction of participation: item 23,“Stopped you doing the things you want to do” (social domain), and item 31, “interfered with your life in general” (emotional domain). Social and emotional domains and the IVI total score were recalculated, excluding responses to these items to assess their relationship with these global measures of restriction of participation. The Spearman correlations between these measures and total and domain scores ranged from 0.44 to 0.82 and were the highest of all correlations assessed
(Table 5) .
When all 32 IVI items were used in principal-components analyses, only 27 participants had vision-related scores for each item, because many individuals stated that at least one item in the IVI was not an activity in which they participated. In cases in which an item is not applicable to the individual, the data are scored as “missing.” Therefore, principal-components analyses were conducted for the 28 items for which more than 80% of participants said the item was relevant. A total of 86 participants had scores for each of these 28 items. Initial principal-components extraction resulted in identification of five components with eigenvalue greater than 1. The initial principal-components solution was dominated by the first factor, eigenvalue 12.15, which explained 43% of the variation in the data. All items in the initial solution loaded onto the first component at 0.48 or greater
(Table 6) . Components 2, 3, 4, and 5 explained 7.3%, 6.1%, 4.4%, and 4.1% of the variation, respectively. Examination of the scree plot for the initial solution suggested that the data were explained by three components, in that the slope of plotted eigenvalues began to flatten at the fourth component.
Principal-components analysis with varimax rotation specifying extraction of three components yielded two components that tended to confirm provisional IVI domains
(Table 6) . The first component consisted primarily of items provisionally grouped in the emotional domain. Items with the highest and most closely related loadings in the third component were all provisionally grouped in the mobility domain. The second component tended to have a mix of items from the work and leisure, social and consumer interaction, and household and personal care domains. All but two items loaded onto at least one component at greater than 0.45. Five loaded on to two components (items 32, 26, 11, 20, and 16). Items 12 and 21 both loaded onto the second component at 0.43.
Reliability
Seventy-five study participants also participated in reliability studies of the IVI, 22 with two IVI questionnaires administered 1 to 2 weeks apart (test-retest), 22 with two different interviewers administering the IVI, and 31 with interviewer and self-administrations. For most items in the IVI, the difference between item responses was within 1 step in 85% or more cases, for each of the three forms of administration
(Table 7) . The mean absolute difference between items was 1 or less for most items in each of the three forms of administration.
For each form of administration, the Guttman split-half correlation between items in each administration was 0.94 for the test-retest and interviewer-interviewer subgroups and 0.95 for the interviewer-self-administered subgroup
(Table 8) . Only the items in the social domain in the test-retest subgroup (
r = 0.73) and the leisure domain in the interviewer-self-administered subgroup (
r = 0.75) had correlation of less than 0.80 between administrations. In the leisure domain, the most common discrepancies between interviewer and self-administered responses were for items 1 (work) and 4 (sport), for which interviewers recorded that the participant did not engage in the activity and the participant recorded that vision did not interfere with the activity (53%, 9/17 and 71%, 10/14, respectively). There were no consistent patterns of discrepancy in responses to other questions in the social or leisure domains. The intraclass correlation coefficients ranged from 0.58 to 0.91 with standard errors of measurement ranging from 0.31 to 0.82.
The mean absolute difference in total and domain scores was less than 1 for each of the domains and the total IVI score for each of the administrations
(Table 8) . The upper limit of confidence limits around the absolute difference in total and domain scores was less than 1, with the exception of the work and leisure domain, in all forms of administration, and the household and personal care domain, in the interviewer-self-administration. For these, the upper confidence limits were 1.09 to 1.14.
Discussion
The purpose of the development of the IVI was to create an instrument that captured a person’s experience with restriction of participation in five functional life domains due to impaired vision. Questions were therefore framed to solicit responses regarding activities that were important to an individual rather than to determine ability to engage in an activity or visual symptoms. Typical of people who have impaired vision in industrialized countries, participants in this study were older adults.
29 30 31 32 33 Age-related macular degeneration followed by glaucoma is a predominant cause of vision loss in industrialized countries. Subanalyses of the data, excluding data for people who participated in self-help groups, did not appreciably alter any of the results, and comparisons of the IVI results between the support group members, patients at the hospital, and the clients of the rehabilitation agency also did not yield appreciable differences. Therefore, the results do not appear to be biased by the coping skills acquired as result of vision support interventions and should be generalizable to the population with impaired vision. However, the participants do not fully represent the adult population of Australia, because participation was limited to people who spoke English sufficiently to comprehend and respond to the questions in the IVI. Studies are under way to evaluate the IVI for use with people who use languages other than English.
Item Characteristics and Scoring
For the IVI to be a practical tool to assess vision rehabilitation needs, it had to be concise but still contain sufficient items to cover adequately the common issues raised in focus groups conducted at the initiation of the study. To achieve the requirement of being a concise tool, it was necessary to reduce the number of items from the original 76 questions.
Most items remaining in the IVI were relevant to more than 80% of participants. Four items (1, 4, 5, and 18) were found to be relevant by less than 80% of the participants. Two of these were closely related in content: “taking part in sports” and “going out to sporting events, movies, and plays.” “Paid or voluntary work” and “traveling or using transport” also were relevant for less than 80% of participants. All who responded that these items were not relevant were older on average than those who deemed them relevant, perhaps signaling that in older adults much of life’s activities are centered around the home. We hesitated to eliminate these items, because the IVI is intended for use in all adults, including those of working age. Therefore, interpretation of the average score for the provisional work and leisure domain should proceed cautiously for older adults, because the number of items that are relevant may be limited.
In earlier versions of the instrument, redundant items or those with floor and ceiling effects were identified and were removed from the questionnaire.
25 26 Most remaining items exhibit at most only a modest correlation with each other. Therefore, IVI items contain unique information with little content overlap. Sequential elimination of items from internal consistency analyses did not affect internal consistency of the IVI, thus demonstrating that the provisional domains and total score are not weighted to a particular item and that each item has a similar contribution to the score. All items contained in the IVI have a greater than 0.20 correlation with the total score—a suggested criterion for retention of items in a scale.
34 All but two remaining items had responses across the full range, suggesting that the IVI items have good discriminative ability to describe a wide range of experience with vision impairment.
Validity
The impact of impaired vision on a person’s ability to participate in society and activities is wholly dependent on the person’s perception of the difficulty experienced with important activities. As such, there is not a gold standard by which to judge the criterion validity of instruments that attempt to measure this subjective response. Evaluation of the validity of these instruments is limited to face and construct validity.
Face validity of the IVI was assured by use of focus groups in its development to ensure that the range of items included in the IVI were relevant in both breadth and content of vision-related issues important to people with impaired vision.
24 Development of the item bank used in the IVI was based primarily on a previously validated questionnaire, providing further assurance that the IVI contained valid content and question format that captured the issues identified by focus groups.
16
In evaluating construct validity of the IVI, we expected that the degree of vision-related reported difficulty with IVI items would have some relationship to actual function-based measures of vision. Although the correlation of the IVI with distance vision was generally modest, all provisional domains and the total score demonstrated a trend of increased reported difficulty with reduced level of vision. Similarly, all domains excepting the leisure domain demonstrated similar relationships with near vision. In other studies low correlation has also been reported with function-based measures of vision and ability to complete activities of daily living and self-reported visual difficulty with tasks.
1 2 IVI domain and total scores correlated most strongly with vision-specific global measures of restriction of participation, suggesting that it elicits responses consistent with the hypothesized construct.
The construct validity of the IVI would be compromised if it were actually measuring the impact of disability from other causes. Participants in this study were older adults and, typical of this age group, many reported that they had other health conditions. Mean IVI scores for each domain and the total did not vary by self-reported other health conditions nor did they vary by reported difficulty experienced because of other health conditions. IVI domain and total scores also did not correlate significantly with age, the physical summary scale (PCS12) of the SF-12, or the self-reported level of general health. Correlation of the IVI with the mental summary scale (MCS12) is difficult to interpret. It might be expected that increasing difficulty with IVI items would correlate with increasing difficulty on the MCS12. We found the opposite and suggest that the correlation may be spurious. However, the general noncorrelation between the IVI and other factors supports that the response to items contained in the IVI is specific to experience with impaired vision.
To further explore the construct validity of the IVI, we used principal-components analysis to explore the underlying structure of the IVI. In principal-components analysis, all items in the IVI loaded onto the first component of the initial solution, adding further support that the items contained in the IVI are measuring one underlying theme. Principal-components analysis did not confirm the five provisional domains identified a priori in the study. However, it tended to confirm the emotional reaction to vision loss and the mobility domains. In the final three-component model, emotional-reaction items loaded heavily onto the first component, suggesting that the underlying structure of the IVI is heavily weighted by these concerns.
Items in the provisional leisure and work, social and consumer interaction, and household and personal care domains seem to tap a related group of issues. These results indicate that combining items from these three domains may in fact give a better picture of the restriction of participation experienced in an older population.
In a practical sense, the implication of the results of principal-components analysis for vision rehabilitation suggest that three areas of restriction of participation may be addressed in assessing needs for intervention. The first and third components suggest assessment of the need for interventions targeted at emotional support and mobility, respectively. The second component is most heavily weighted to vision-dependent or vision-critical activities, such as viewing television or reading. This suggests a need for vision-enhancement intervention.
Our principal-component analysis was limited to 28 of the 32 items in the IVI because of inadequate sample size to include four of the items. As have others,
7 we find that the resultant data set is not entirely representative of the complete sample. The 22 individuals whose data were not included in the principal-components analysis tended to be somewhat older on average—77 compared with 71 years of age. They did not differ on level of vision, cause of vision loss, or reported other illnesses. Given that age was unrelated to either domain or total IVI score, we expect that the effect of excluding data for these older individuals from the principal-components analysis would be minimal. However, we are still unable to evaluate these four items in terms of their role in the underlying structure of the IVI.
Reliability
Test-retest reliability is particularly important if the IVI is to be used to measure change in limitations of participation over time or as a measure of low vision service outcome. Our results compare favorably with results reported for the VCM1, in which the interclass correlation coefficient was 0.90.
16 Participants in this part of the research had not received vision support or rehabilitation at either the hospital or at a vision rehabilitation agency. Thus, it is unlikely that the test-retest results were affected by intervention. Upper limits of the absolute difference suggest the IVI has the ability to discriminate changes in IVI score of 1 or more for most of the provisional domains and the total IVI score.
The consistency of the IVI between different forms of administration gives it flexibility in application. For example, the IVI could be completed by a person before a clinic visit, with results comparable to administration of the IVI by clinic personnel. Also, if the IVI is used as an outcome measure, there is some assurance that changes before and after administration are not due to differences in forms of administration. It should be noted that self-administered reliability comparisons apply only to people with better vision and that other forms of self-administration have not been evaluated.
Conclusions
The results indicate that the IVI has sufficient internal and construct validity to measure the effect of vision impairment on restriction of participation in daily activities. The IVI has acceptable reliability over a short period and yields consistent results between forms of administration. It provides information important for determining vision specific restriction of participation that is not captured in currently used clinical measures in the rehabilitation setting. It is a practical, easy to administer tool that can be used in a vision rehabilitation setting to indicate vision intervention needs. Importantly, it can be used with older people, the population group most likely to develop impaired vision. Its stability over time indicates that it has potential to evaluate outcomes of intervention.
Supported by The Victorian Health Promotion Foundation, The Vision Australia Foundation, and The Jack Brockhoff Foundation, Melbourne, Australia.
Submitted for publication April 13, 2001; revised October 16, 2001; accepted November 19, 2001.
Commercial relationships policy: N.
The publication costs of this article were defrayed in part by page charge payment. This article must therefore be marked “
advertisement” in accordance with 18 U.S.C. §1734 solely to indicate this fact.
Corresponding author: LeAnn M. Weih, Centre for Eye Research Australia, Royal Victorian Eye and Ear Hospital, 32 Gisborne Street, Locked Bag 8, East Melbourne, Victoria, Australia 8002;
leann@unimelb.edu.au. Table 1. Demographic Characteristics of Study Participants
Table 1. Demographic Characteristics of Study Participants
| Age (y) | |
| Range | 25–98 |
| Mean± SD | 72 ± 14 |
| Median | 75 |
| Gender | |
| Female | 70 (61) |
| Male | 45 (39) |
| Distance visual acuity | |
| Field loss with visual acuity ≥ 6/12 | 21 (18) |
| <6/12 to 6/18 | 39 (34) |
| <6/18 to 6/60 | 33 (29) |
| <6/60 | 21 (18) |
| Near Vision | |
| N8 | 36 (41) |
| N20 | 36 (41) |
| N48 or less | 15 (18) |
| Main cause of vision loss | |
| Age-related maculopathy | 42 (39) |
| Glaucoma | 24 (22) |
| Diabetic retinopathy | 16 (15) |
| Other (cataract, other retinopathy) | 26 (24) |
| Duration of vision impairment | |
| Range | <1 year to 74 |
| Mean± SD | 12 ± 17 |
| Median | 4 |
| Hearing loss | |
| Yes | 14 (29) |
| No | 34 (71) |
| Comorbidity | |
| Yes | 78 (85) |
| No | 14 (15) |
| PCS12 | |
| Range | 39.5–60.8 |
| Mean± SD | 39.5 ± 12.4 |
| Median | 39.1 |
| MCS12 | |
| Range | 23.4–68.8 |
| Mean ± SD | 49.8 ± 11.7 |
| Median | 52.7 |
Table 2. Questionnaire Items and Scoring Characteristics
Table 2. Questionnaire Items and Scoring Characteristics
| Domain | Item | Content | n | Relevant Item | | Responses >0 | | Range of Responses | | Mean Score ± SD |
| | | | n | % | n | % | Min. | Max. | |
| L | 1 | Paid or voluntary work? | 115 | 41 | 36 | 29 | 70 | 0 | 5 | 2.61 ± 2.01 |
| L | 2 | Favorite pastimes or hobbies? | 115 | 113 | 98 | 97 | 86 | 0 | 5 | 3.08 ± 1.70 |
| L | 3 | Ability to see and enjoy television? | 115 | 112 | 97 | 92 | 82 | 0 | 5 | 2.28 ± 1.53 |
| L | 4 | Taking part in sporting activities? | 115 | 83 | 72 | 57 | 69 | 0 | 5 | 2.25 ± 1.94 |
| L | 5 | Going out to sports events, movies, or plays? | 115 | 65 | 57 | 48 | 74 | 0 | 5 | 2.89 ± 2.05 |
| S | 6 | Shopping? | 115 | 105 | 91 | 83 | 79 | 0 | 5 | 2.54 ± 1.66 |
| S | 7 | Reading ordinary-sized print? | 115 | 115 | 100 | 106 | 92 | 0 | 5 | 3.81 ± 1.50 |
| S | 8 | Visiting friends or family? | 115 | 107 | 93 | 43 | 40 | 0 | 5 | 1.03 ± 1.50 |
| S | 9 | Recognizing or meeting people? | 115 | 115 | 100 | 88 | 77 | 0 | 5 | 2.64 ± 1.77 |
| S | 10 | Getting information that you need? | 115 | 115 | 100 | 74 | 64 | 0 | 5 | 2.01 ± 1.77 |
| H | 11 | Generally looking after your appearance? | 115 | 115 | 100 | 67 | 58 | 0 | 5 | 1.47 ± 1.52 |
| H | 12 | Opening packaging? | 115 | 111 | 97 | 52 | 47 | 0 | 5 | 1.18 ± 1.50 |
| H | 13 | Reading labels or instructions on medicines? | 115 | 111 | 97 | 95 | 86 | 0 | 5 | 3.18 ± 1.76 |
| H | 14 | Operating household appliances and the telephone? | 115 | 112 | 97 | 65 | 58 | 0 | 5 | 1.39 ± 1.47 |
| M | 15 | Reading a sign across the street? | 114 | 112 | 97 | 101 | 90 | 0 | 5 | 3.58 ± 1.68 |
| M | 16 | Getting about outdoors? | 115 | 108 | 94 | 83 | 77 | 0 | 5 | 2.30 ± 1.67 |
| M | 17 | Made you go carefully to avoid falling or tripping? | 115 | 111 | 97 | 94 | 85 | 0 | 5 | 2.67 ± 1.49 |
| M | 18 | Interfered with travelling or using transport? | 115 | 91 | 80 | 68 | 75 | 0 | 5 | 2.34 ± 1.84 |
| M | 19 | Going down steps, stairs, or curbs? | 115 | 109 | 95 | 90 | 83 | 0 | 5 | 2.41 ± 1.45 |
| H | 20* | Your general safety at home? | 115 | 115 | 100 | 53 | 46 | 0 | 4 | 0.92 ± 1.22 |
| H | 21 | Spilling or breaking things? | 115 | 115 | 100 | 58 | 50 | 0 | 4 | 1.13 ± 1.33 |
| M | 22* | Your general safety when out of your home? | 115 | 115 | 100 | 79 | 69 | 0 | 5 | 1.82 ± 1.56 |
| S | 23* | Stopped your doing the things you want to do? | 115 | 115 | 100 | 95 | 83 | 0 | 5 | 2.42 ± 1.57 |
| S | 24 | Needed help from other people? | 115 | 115 | 100 | 94 | 82 | 0 | 5 | 2.28 ± 1.58 |
| E | 25* | Felt embarrassed? | 114 | 114 | 100 | 52 | 46 | 0 | 5 | 1.13 ± 1.45 |
| E | 26* | Felt frustrated or annoyed? | 114 | 114 | 100 | 92 | 81 | 0 | 5 | 2.46 ± 1.59 |
| E | 27* | Have you felt lonely or isolated? | 114 | 114 | 100 | 50 | 44 | 0 | 5 | 1.07 ± 1.41 |
| E | 28* | Have you felt sad or low? | 114 | 114 | 100 | 66 | 58 | 0 | 5 | 1.49 ± 1.51 |
| E | 29* | Worried about your eyesight getting worse? | 114 | 114 | 100 | 89 | 78 | 0 | 5 | 2.37 ± 1.72 |
| E | 30* | Concerned or worried about coping with everyday life? | 114 | 114 | 100 | 83 | 73 | 0 | 5 | 1.86 ± 1.57 |
| E | 31* | Interfered with your life in general? | 114 | 114 | 100 | 93 | 82 | 0 | 5 | 2.41 ± 1.56 |
| E | 32 | Felt like a nuisance or a burden? | 113 | 113 | 100 | 57 | 50 | 0 | 5 | 1.40 ± 1.61 |
Table 3. Items Correlated at 0.60 or Greater
Table 3. Items Correlated at 0.60 or Greater
| Item 1 | Item 2 | n Cases, † | Spearman’s r |
| 1 | 2 | 41 | 0.76 |
| 1 | 23 | 41 | 0.61 |
| 5 | 6 | 64 | 0.66 |
| 7 | 13 | 111 | 0.61 |
| 10 | 16 | 108 | 0.60 |
| 16 | 17 | 105 | 0.65 |
| 16 | 21 | 108 | 0.68 |
| 23* | 31* | 114 | 0.69 |
| 27* | 28* | 114 | 0.61 |
| 28* | 31* | 114 | 0.62 |
Table 4. Internal Consistency of Provisional Domains
Table 4. Internal Consistency of Provisional Domains
| Domain | Number of Cases* | Number of Items | Cronbach α |
| Leisure | 29 | 5 | 0.88 |
| Household | 106 | 6 | 0.80 |
| Social | 99 | 7 | 0.85 |
| Mobility | 89 | 6 | 0.89 |
| Emotional | 113 | 8 | 0.88 |
| Total | 27 | 32 | 0.96 |
Table 5. Correlation of Average Domain and Total Score with Demographic Characteristics
Table 5. Correlation of Average Domain and Total Score with Demographic Characteristics
| Domain | Age | Duration of Vision Loss | Interferes with General Life | Stops from Doing Things | Distance Visual Acuity | Near Visual Acuity | PCS12 | MCS12 | General Health |
| Leisure | 0.056 | −0.064 | 0.63, † | 0.63, † | 0.244, † | 0.128 | −0.128 | −0.287, † | 0.261, † |
| Household | 0.023 | −0.101 | 0.47, † | 0.50, † | 0.209* | 0.209* | −0.025 | −0.171* | 0.022 |
| Social | −0.007 | −0.133* | 0.64, † | 0.70, † | 0.311, † | 0.297, † | −0.022 | −0.269* | −0.100 |
| Mobility | 0.011 | −0.044 | 0.44, † | 0.53, † | 0.251, † | 0.245, † | −0.042 | −0.166* | −0.005 |
| Emotional | 0.019 | −0.152* | 0.82, † | 0.72, † | 0.191, † | 0.183* | 0.015 | −0.370, † | −0.136 |
| Total | 0.025 | −0.116 | 0.73, † | 0.74, † | 0.306, † | 0.306, † | −0.035 | −0.299, † | −0.102 |
Table 6. Principal Components of the IVI
Table 6. Principal Components of the IVI
| Item/Domain | Content | Unrotated PCA Model Item Loading | Varimax-Rotated PCA Model | | |
| | | Factor 1 Item Loading | Factor 2 Item Loading | Factor 3 Item Loading |
| 2L | Favorite pastimes or hobbies? | 0.58 | | 0.48 | |
| 3L | Ability to see and enjoy television? | 0.66 | | 0.67 | |
| 25E | Felt embarrassed? | 0.62 | 0.72 | | |
| 32E | Felt like a nuisance or a burden? | 0.74 | 0.59 | | 0.45 |
| 26E | Felt frustrated or annoyed? | 0.73 | 0.58 | 0.54 | |
| 27E | Have you felt lonely or isolated? | 0.63 | 0.69 | | |
| 28E | Have you felt sad or low? | 0.69 | 0.78 | | |
| 29E | Worried about your eyesight getting worse? | 0.49 | 0.66 | | |
| 31E | Interfered with your life in general? | 0.76 | 0.80 | | |
| 30E | Concerned or worried about coping with everyday life? | 0.68 | 0.71 | | |
| 21H | Spilling or breaking things? | 0.59 | | | |
| 11H | Generally looking after your appearance? | 0.68 | 0.49 | 0.51 | |
| 12H | Opening packaging? | 0.62 | | | |
| 13H | Reading labels or instructions on medicines? | 0.55 | | 0.70 | |
| 14H | Operating household appliances and the telephone? | 0.73 | | 0.56 | |
| 20H | Your general safety at home? | 0.69 | 0.52 | | 0.45 |
| 24S | Needed help from other people? | 0.73 | | 0.59 | |
| 6S | Shopping? | 0.69 | | 0.60 | |
| 7S | Reading ordinary-size print? | 0.56 | | 0.78 | |
| 8S | Visiting friends or family? | 0.58 | 0.45 | | |
| 9S | Recognizing or meeting people? | 0.68 | | 0.68 | |
| 10S | Getting information that you need? | 0.72 | | 0.58 | |
| 23S | Stopped you doing the things you want to do? | 0.76 | 0.64 | | |
| 15M | Reading a sign across the street? | 0.65 | | 0.62 | |
| 16M | Getting about outdoors? | 0.75 | | 0.50 | 0.71 |
| 17M | Made you go carefully to avoid falling or tripping? | 0.53 | | | 0.78 |
| 22M | Your general safety when out of your home? | 0.67 | | | 0.72 |
| 19M | Going down steps, stairs, or curbs? | 0.54 | | | 0.76 |
| Percent Variation Explained | | 43.39 | 43.39 | 7.73 | 6.11 |
Table 7. Difference in Item Scores between Administrations
Table 7. Difference in Item Scores between Administrations
| Item | Content | Test-Retest | | Interviewer-Interviewer | | Interviewer-Self | |
| | Mean Absolute Difference | % ≤1 Step Difference | Mean Absolute Difference | % ≤1 Step Difference | Mean Absolute Difference | % ≤1 Step Difference |
| 1 | Paid or voluntary work? | 0.50 | 85 | 0.57 | 86 | 2.72 | 52 |
| 2 | Favorite pastimes or hobbies? | 0.80 | 75 | 0.76 | 81 | 1.52 | 56 |
| 3 | Ability to see and enjoy television? | 0.75 | 85 | 1.04 | 81 | 1.24 | 72 |
| 4 | Taking part in sporting activities? | 1.60 | 65 | 1.09 | 67 | 1.24 | 64 |
| 5 | Going out to sports events, movies, or plays? | 0.95 | 85 | 0.90 | 90 | 1.64 | 72 |
| 6 | Shopping? | 1.15 | 75 | 0.52 | 95 | 0.84 | 76 |
| 7 | Reading ordinary-sized print? | 0.50 | 85 | 0.24 | 95 | 0.72 | 84 |
| 8 | Visiting friends or family? | 0.85 | 85 | 0.71 | 86 | 1.32 | 64 |
| 9 | Recognizing or meeting people? | 0.75 | 80 | 1.04 | 71 | 0.88 | 80 |
| 10 | Getting information that you need? | 1.00 | 70 | 1.09 | 76 | 1.25 | 71 |
| 11 | Generally looking after your appearance? | 0.45 | 90 | 0.66 | 86 | 0.84 | 76 |
| 12 | Opening packaging? | 0.60 | 95 | 0.86 | 76 | 1.28 | 60 |
| 13 | Reading labels or instructions on medicines? | 0.85 | 90 | 0.57 | 86 | 1.08 | 84 |
| 14 | Operating household appliances and the telephone? | 0.75 | 80 | 0.62 | 86 | 1.12 | 72 |
| 15 | Reading a sign across the street? | 0.65 | 95 | 0.86 | 81 | 1.00 | 76 |
| 16 | Getting about outdoors? | 0.95 | 80 | 1.00 | 86 | 0.76 | 88 |
| 17 | Made you go carefully to avoid falling or tripping? | 0.90 | 80 | 0.76 | 76 | 1.25 | 71 |
| 18 | Interfered with traveling or using transport? | 0.90 | 90 | 0.67 | 86 | 1.20 | 72 |
| 19 | Going down steps, stairs, or curbs? | 1.05 | 85 | 0.81 | 76 | 0.84 | 76 |
| 20 | Your general safety at home? | 0.72 | 89 | 0.81 | 76 | 0.91 | 78 |
| 21 | Spilling or breaking things? | 0.79 | 84 | 0.57 | 91 | 0.77 | 86 |
| 22 | Your general safety when out of your home? | 0.70 | 95 | 1.09 | 81 | 0.96 | 68 |
| 23 | Stopped you doing the things you want to do? | 0.85 | 75 | 0.76 | 91 | 0.91 | 75 |
| 24 | Needed help from other people? | 0.70 | 90 | 0.86 | 76 | 0.64 | 92 |
| 25 | Felt embarrassed? | 0.55 | 85 | 0.47 | 91 | 0.56 | 80 |
| 26 | Felt frustrated or annoyed? | 0.75 | 85 | 0.76 | 86 | 0.88 | 76 |
| 27 | Have you felt lonely or isolated? | 0.55 | 90 | 0.43 | 91 | 0.56 | 84 |
| 28 | Have you felt sad or low? | 0.35 | 100 | 0.33 | 91 | 0.76 | 76 |
| 29 | Worried about your eyesight getting worse? | 0.68 | 90 | 0.62 | 91 | 1.00 | 68 |
| 30 | Concerned or worried about coping with everyday life? | 0.79 | 84 | 1.00 | 71 | 0.84 | 80 |
| 31 | Interfered with your life in general? | 0.68 | 84 | 0.91 | 76 | 0.84 | 80 |
| 32 | Felt like a nuisance or a burden? | 0.74 | 79 | 0.71 | 81 | 0.72 | 80 |
Table 8. Guttman Split-Half Correlation, Intraclass Correlation, and Absolute Difference of Domains between Forms of Administration
Table 8. Guttman Split-Half Correlation, Intraclass Correlation, and Absolute Difference of Domains between Forms of Administration
| n | Guttman Split-Half Correlation (range, 0–1) | Intraclass Correlation Cofficient (range, 0–1) | Standard Error of Measurement | n | Mean Absolute Difference ± SD; (95% CI) |
| Test-Retest | | | | | | |
| Total | 17 | 0.94 | 0.88 | 0.31 | 20 | 0.30 ± 0.32 (0.15–0.45) |
| Work and leisure | 20 | 0.82 | 0.65 | 0.79 | 20 | 0.69 ± 0.89 (0.26–1.10) |
| Household and personal care | 18 | 0.87 | 0.84 | 0.40 | 20 | 0.41 ± 0.39 (0.23–0.59) |
| Mobility | 20 | 0.85 | 0.71 | 0.46 | 20 | 0.56 ± 0.57 (0.29–0.83) |
| Consumer and social interaction | 20 | 0.73 | 0.83 | 0.49 | 20 | 0.47 ± 0.48 (0.24–0.69) |
| Emotional reaction to vision loss | 19 | 0.94 | 0.83 | 0.44 | 20 | 0.42 ± 0.46 (0.21–0.64) |
| Interviewer-interviewer | | | | | | |
| Total | 20 | 0.94 | 0.91 | 0.29 | 21 | 0.32 ± 0.26 (0.20–0.44) |
| Work and leisure | 20 | 0.91 | 0.78 | 0.73 | 21 | 0.77 ± 0.69 (0.46–1.09) |
| Household and personal care | 21 | 0.93 | 0.93 | 0.30 | 21 | 0.34 ± 0.25 (0.23–0.46) |
| Mobility | 21 | 0.88 | 0.75 | 0.53 | 21 | 0.54 ± 0.72 (0.31–0.78) |
| Consumer and social interaction | 21 | 0.82 | 0.81 | 0.60 | 21 | 0.43 ± 0.55 (0.17–0.67) |
| Emotional reaction to vision loss | 21 | 0.92 | 0.92 | 0.27 | 21 | 0.32 ± 0.21 (0.22–0.42) |
| Interviewer-Self | | | | | | |
| Total | 22 | 0.95 | 0.84 | 0.48 | 25 | 0.47 ± 0.49 (0.26–0.67) |
| Work and leisure | 25 | 0.75 | 0.73 | 0.81 | 25 | 0.80 ± 0.83 (0.46–1.14) |
| Household and personal care | 22 | 0.86 | 0.58 | 0.82 | 25 | 0.73 ± 0.92 (0.34–1.10) |
| Mobility | 24 | 0.91 | 0.78 | 0.62 | 25 | 0.66 ± 0.59 (0.41–0.90) |
| Consumer and social interaction | 24 | 0.91 | 0.81 | 0.60 | 25 | 0.65 ± 0.61 (0.35–0.85) |
| Emotional reaction to vision loss | 25 | 0.89 | 0.81 | 0.55 | 25 | 0.53 ± 0.57 (0.29–0.77) |
Klein BE, Klein R, Jensen SC. A short questionnaire on visual function in older adults to supplement ophthalmic examination.
Am J Ophthalmol. 2000;130:350–352.
[CrossRef] [PubMed]Haymes SA, Johnston AW, Heyes AS. The development of the Melbourne Low Vision ADL Index: a measure of vision disability.
Invest Ophthalmol Vis Sci. 2001;42:1215–1225.
[PubMed]Friedman SM, Munoz B, Rubin GS, West SK, Bandeen-Roche K, Fried LP. Characteristics of discrepancies between self-reported visual function and measured reading speed.
Invest Ophthalmol Vis Sci. 1999;40:858–864.
[PubMed]Newman D, Houser B. Visual disability inventory: documenting functional impairment caused by cataract.
J Cataract Refract Surg. 1991;17:244–245.
[CrossRef] [PubMed]Mangione CM, Berry S, Spritzer K, et al. Identifying the content area for the 51-item National Eye Institute Visual Function Questionnaire.
Arch Ophthalmol. 1998;116:227–233.
[PubMed]Mangione C, Phillips R, Seddon J, et al. Development of the Activities of Daily Vision Scale.
Med Care. 1992;30:1111–1126.
[CrossRef] [PubMed]Steinberg E, Tielsch J, Schein O, et al. An index of functional impairment in patients with cataract.
Arch Ophthalmol. 1994;112:630–638.
[CrossRef] [PubMed]Pesudovs K, Coster D. An instrument for assessment of subjective visual disability in cataract patients.
Br J Ophthalmol. 1998;82:617–624.
[CrossRef] [PubMed]Worrall L, Hickson L, Barnett H, Lovie-Kitchin J. The performance of older people on everyday visual tasks.
Clin Exp Optom. 1993;76:127–135.
[CrossRef] Rubin G, Bandeen-Roche D, Prasada-Rao P, Fried L. Visual impairment and disability in older adults.
Optom Vis Sci. 1994;71:750–760.
[CrossRef] [PubMed]Nieuwenhuijsen E, Frey W, Crews J. Measuring small gains using the ICIDH severity of disability scale: assessment practice among older people who are blind.
Int Disability Studies. 1991;13:29–33.
[CrossRef] Long R. Functional Independence Measure for Blind Adults (FIMBA). 1993; Veteran Affairs Rehabilitation Research and Development Center Atlanta, GA.
Ross J, Bron A, Clarke D. Contrast sensitivity and visual disability in chronic simple glaucoma.
Br J Ophthalmol. 1984;68:821–827.
[CrossRef] [PubMed]Szlyk J, Arditi A, Coffey Bucci P, Laderman D. Self-report in functional assessment of low vision. J Vis Impair Blindness. 1990;84:61–66.
Wolffsohn JS, Cochrane AL. Design of the low vision quality of life questionnaire (LVQOL) and measuring the outcome of low-vision rehabilitation.
Am J Ophthalmol. 2000;130:793–802.
[CrossRef] [PubMed]Frost NA, Sparrow JM, Durant JS, et al. Development of a questionnaire for measurement of vision related quality of life.
Ophthalmic Epidemiol. 1998;5:185–210.
[CrossRef] [PubMed]World Health Organization. International Classification of Impairments, Activities, and Participation: Beta-1 Draft for Field Trials. 1997; World Health Organization Geneva.
Ivers RQ, Cummins RG, Mitchell P, Attebo K. Visual impairment and falls in older adults: the Blue Mountains Eye Study.
J Am Geriatr Soc. 1998;46:58–64.
[CrossRef] [PubMed]West SK, Munoz B, Rubin GS, et al. Function and visual impairment in a population-based study of older adults.
Invest Ophthalmol Vis Sci. 1997;38:72–82.
[PubMed]Wang JJ, Mitchell P, Smith W, Cumming RG, Attebo K. Impact of visual impairment on use of community support services by elderly persons: the Blue Mountains Eye Study.
Invest Ophthalmol Vis Sci. 1999;40:12–19.
[PubMed]Klein BE, Klein R, Lee KE, Cruikshanks KJ. Performance-based and self-assessed measures of visual function as related to history of falls, hip fractures, and measured gait time: The Beaver Dam Eye Study.
Ophthalmology. 1998;105:160–164.
[CrossRef] [PubMed]Weih LM, McCarty CA, Taylor HR. Functional implications of vision impairment.
Clin Exp Ophthalmol. 2000;28:153–155.
[CrossRef] Ware JE, Kosinski M, Keller SD. SF-12: How to Score the SF-12 Physical and Mental Health Summary Scales. 1998; 3rd ed. Quality Metric Inc. Lincoln, RI.
Keeffe JE, Lam D, Cheung A, Dinh T, McCarty CA. Impact of vision impairment on functioning.
Aust NZ J Ophthalmol. 1998;26:S16–S18.
[CrossRef] Keeffe JE, McCarty CA, Hassell JB, Gilbert AG. Description and measurement of handicap caused by vision impairment.
Aust NZ J Ophthalmol. 1999;27:184–186.
[CrossRef] Hassell JB, Weih LM, Keeffe JE. A measure of handicap for low vision rehabilitation: the impact of vision impairment profile.
Clin Exp Ophthalmol. 2000;28:156–161.
[CrossRef] Fleiss JL. Design and Analysis of Experiments. 1986; John Wiley & Sons New York.
Tabachnick B, Fidell L. Using Multivariate Statistics. 1996; HarperCollins College Publishers Inc New York.
Sommer A, Tielsch JM, Katz J, et al. Racial differences in the cause-specific prevalence of blindness in East Baltimore.
N Engl J Med. 1991;325:1412–1417.
[CrossRef] [PubMed]Dana MR, Tielsch JM, Enger C, et al. Visual impairment in a rural Appalachian community: prevalence and causes.
JAMA. 1990;264:2400–2405.
[CrossRef] [PubMed]Klein R, Klein BEK, Linton KLP, De Mets DL. The Beaver Dam Eye Study: visual acuity.
Ophthalmology. 1991;98:1310–1315.
[CrossRef] [PubMed]Attebo K, Mitchell P, Smith W. Visual acuity and the causes of visual loss in Australia: the blue Mountains Eye Study.
Ophthalmology. 1996;103:357–364.
[CrossRef] [PubMed]Weih LM, VanNewkirk MR, McCarty CA, Taylor HR. Age specific causes of bilateral visual impairment.
Arch Ophthalmol. 2000;118:264–269.
[CrossRef] [PubMed]Streiner D, Norman G. Health Measurement Scales. 1995; Oxford University Press Oxford, UK.