Specificity is the ability of a test to identify correctly those who do not have the disease, that is, the percentage of "true negatives" among those not diseased. In the above question, there are 600 true negatives among 800 who are not diseased. Hence, the specificity is (600/800)x100 = 75 per cent. Ref: Park's Textbook Of Preventive And Social Medicine, By K. Park, 19th Edition, Pages 118, 119.