Presenting (habitual) visual acuity was assessed using the Early Treatment Diabetic Retinopathy Study (ETDRS) chart, and ETDRS refraction was performed on participants worse than 20/30.
15 Visual acuity was obtained under normal luminance with illuminated ETDRS chart (Lighthouse-illuminated box; Lighthouse International, New York, NY). Presenting and best-corrected binocular distance visual acuity was converted to logarithm of the minimum angle of resolution (logMAR). Functional status of ADLs and IADLs were measured using the standardized validated questionnaires.
10,11 ADL assessments included five items: difficulty getting out of bed or a chair; difficulty dressing yourself; difficulty bathing or showering; difficulty using toilet; and difficulty feeding yourself (e.g., cutting your food or drinking from a glass). IADL assessments included six items: difficulty using the phone; difficulty doing light housework (sweeping, doing dishes) or light yard work (watering); difficulty doing heavy housework (scrubbing floors, washing windows) or heavy yard work (raking, gardening); difficulty preparing your own meals; difficulty managing your own money; difficulty shopping for personal items, such as medicines. Each question always started with: “By yourself, that is without help of another person or special equipment, do you have any difficulty … ?” Each question had one of the following possible answers: “no difficulty,” “a little difficulty,” “some difficulty,” “a lot of difficulty,” “unable to do this for health or physical reasons.” We used confirmatory factor analysis (CFA) and associated model fit statistics to validate the items in the IADL and ADL scales. Both global model fit tests and indices (χ
2 test of model fit, root mean square of approximation [RMSEA], etc.) and component fit such as factor loadings and their statistical significance were used. To facilitate analysis, disability scores of ADLs and IADLs were constructed by summing the five and six items, respectively, at each time point. The sum scores were subsequently used in the trajectory models. Means and standard deviations of these scores for the sample are provided in
Table 1. Because the scales are outcomes in the trajectory models, the random measurement error is part of the regression error and therefore does not impact the association parameters.
Control variables include demographics, physical health conditions, severe depression, and health behavior–related variables. A standardized form was used to query all participants about demographics (e.g., age, sex, race, formal education) and medical history of physical health conditions. All control variables, except age, used in models were measured at the baseline assessment. Age was allowed to be time-varying in order to assess health trajectories with respect to age. Education was measured as highest grade completed and ranged from 0 to 17. Medical history included 15 medical conditions that were self-reported responses to the question “Has a doctor ever told you that you have…?” The 15 medical conditions included diabetes, stroke, heart disease, high blood pressure, cancer, asthma, arthritis, angina, back problem, broken hip, congestive heart failure, claudication, emphysema, Meniere's disease, and Parkinson's disease. Severe depression was assessed using the Severe Depression subscale of General Health Questionnaire (GHQ)-28. The questions included phrasing such as: (1) “thought that you might do away with yourself”; (2) “felt that life is entirely hopeless”; (3) “felt that life isn't worth living”; (4) “felt at times you couldn't do anything because nerves were too bad”; (5) “found that the idea of taking your own life kept coming to your mind”; (6) “found yourself wishing you were dead and away from it all”; (7) “been thinking of yourself as a worthless person.” Each question had four possible answers: “not at all,” “no more than usual,” “rather more than usual,” or “much more than usual.” An overall indicator that we call a severe depression scale was created by taking the average value of the seven questions. Health behavior–related variables include questions on smoking measured as current smoker, past smoker, or nonsmoker (reference); alcohol use measured as current user, past user, or nonuser (reference). We measured height and weight and categorized the body mass index (BMI) as normal (reference BMI 18.5 to <25); underweight (BMI <18.5); overweight (BMI 25 to <30); obese (BMI 30–35); or very obese (BMI >35). All of these measures were performed at the baseline assessment.