If blindness is surveyed using schools as compared to population surveys, then estimation of prevalence of blindness will have ?

Blindness situation in India Estimated prevalence of blindness in India (total) : 11.2 per 1000 population. Estimated prevalence of blindness in India (0-14 years) : 0.1 per 1000 population. Estimated prevalence of blindness in India (15-49 years) : 0.6 per 1000 population. Estimated prevalence of blindness in India (50+ years) : 77.3 per 1000 population. So if schools are used where only refractive errors generally constitute blindness (that too very few are actually blind i.e. <6 / 60) As compared to population (where age-related cataract constitute as major cause of blindness), it would lead to underestimation of prevalence of blindness in the country.