A total of 607 subjects were selected at random. Four hundred sixty-seven (76.9%) subjects were contacted successfully, whereas 94 (15.5%) subjects could not be reached after three attempts at different times on different days; 46 (7.6%) telephone numbers were invalid. Among those who were contacted successfully, 300 (64.2%) completed all phases of the interview. Of those without full interview data, 105 (22.5%) subjects were ineligible (mostly due to having already had surgery in the fellow eye or difficulty communicating), 55 (11.8%) did not complete the interview, and 7 (1.5%) refused to take part in the study
(Fig. 1) . Among 456 subjects with valid telephone numbers who were eligible for the interview, the overall response rate was 65.8% completing interviews, 67.6% from hospital A and 64.1% from hospital B
(Fig. 1) .
Among participating subjects, 177 (59.0%) respondents were women, about half (48.2%) were 76 years or older, and 142 (47.5%) had no schooling or had completed only preprimary education. Respondents and nonrespondents did not differ significantly with respect to sex, though nonrespondents had been on the waiting list some 4 months longer than the mean for respondents (17.3 ± 15.1 months,
P = 0.03;
Table 1 ). More than half of respondents (55.9%) had monthly household income of less than US$769 compared with the median Hong Kong monthly household income of US$2212 in 2006.
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Among subjects responding to the question, roughly three fourths (220/294 = 74.8%) were willing to pay anything for cataract surgery. The mean amount willing to pay among such persons was US$552 ± 443
(Table 1) . The maximum acceptable waiting time for nearly half (
n = 131, 43.7%) of respondents was 12 months or less, but about one fifth of respondents were willing to wait more than 3 years for cataract surgery
(Table 1) .
Twenty-eight (9.3%) respondents were considering having cataract surgery in the private sector
(Table 2) . Among them, the purpose of more than 90% was to reduce waiting time. Two hundred seventy-two (90.7%) respondents were not considering having private surgery. For the majority (73.5%), the reason for preferring surgery in the public sector was inability to afford private fees.
Factors potentially affecting willingness to pay for cataract surgery were explored in two separate statistical models. The outcome of the first of these was willingness versus unwillingness to pay something, while the second explored the amount willing to pay among those subjects who would pay something.
In the first model, subjects willing to pay something for cataract surgery were significantly more likely to be willing to wait 12 months or less for surgery than those unwilling to pay anything (
Table 3 ; odds ratio [OR] = 4.34; 95% confidence interval [CI] = 1.75–10.73;
P = 0.002) were more likely to know friends or relatives who had undergone cataract surgery (OR = 2.20; 95% CI = 1.10–4.36;
P = 0.025) and were more likely to use their own savings to pay for the operation (OR = 2.21; 95% CI = 1.05–4.67;
P = 0.038). Age, sex, education level, marital status, family income, time on waiting list, self-reported visual function, impact of vision on work, and hospital site were not significantly associated with willingness to pay
(Table 3) .
In the second model, willingness to pay a larger amount for surgery was associated with considering surgery in the private sector (additional amount willing to pay: US$580 ± 752,
P < 0.0005,
Table 4 ). Subjects who had visual function scores below 80 were willing to pay (US$799 ± 866) significantly more compared with those with scores of 80 or higher (US$607 ± 641,
P < 0.007). Subjects reporting that visual impairment was affecting their work (US$846 ± 642) were willing to pay significantly more than were subjects working without vision problems (US$615 ± 608,
P = 0.04) and subjects who were not working because of vision problems (US$648 ± 833,
P = 0.04). In addition, those who knew friends or relatives who had undergone cataract surgery (additional amount willing to pay: US$133 ± 747,
P = 0.028) or those who use sources other than government benefits to pay for surgery (additional amount willing to pay: US$244 ± 798,
P = 0.043), were willing to pay significantly more. Age, sex, educational level, marital status, time on waiting list, and hospital site were not significantly associated with amount willing to pay. The observed effect of household income did not follow a specific trend
(Table 4) .