The estimation of prevalence of blindness by blind school survey in place of population-based survey would lead to:
Over-estimate
Under-estimate
There would be no difference
Both of the methods are not to be used for this
Ans. c. There would be no differenceAccording to NAB (National Association for Blind), India Legal blindness is defined as visual acuity of not greater than 20/200 in the better eye with best correction or a visual field of less than 20 degrees.What does 20/200 means?A person with normal visual acuity can see an object clearly, at 200 feet; a legally blind person must be 20 feet or closer to see the same object.Note: 20/200 feet is same as 6/60 metersAccording to NPCB, the criteria for blindness (Economic Blindness) is visual acuity of <6/60 in the better eye with best possible correction.Thus, since both the criteria are same, the prevalence will remain same.
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