300 diseased people were made to undergo a test of which only 60 were positive. 40 people were found to be false positive for the test. The positive predictive value of the test is:

Ans: b (60/100)Ref: Park, 20th ed. Pg. 126 19 ed., Pg. 119Screening test resultsDiagnosisTotalDiseasedNot diseasedPositiveNegativea (True positive)c (False negative)b (False positive)d (True negative)a+bc+dTotala+cb+da+b+c+dSensitivity = a/ (a+c) x 1 (X)Specificity = d/ (b+d) x 1 (M)Predictive value of positive test= a/ (a+b) x 100Predictive value of a negative test = d/ (c+d) x 100Percentage of false negatives = c/ (a+c) x 100Percentage of false positives = b/ (b+d) x 100Sensitivity is the ability of a test to identify correctly all those who have the disease that is "true positive". A screening test which is very sensitive has few "false negatives".Specificity is the ability of a test to identify correctly those who do not have the disease that is true negatives. A screening test which is very specific has few "false positives".