Numerator in negative predictive value -
Ans. is 'c' i.e., True negative o The results of a screening test can be arranged in following table : -Screening test resultsDiagnosisTotalDiseasedNot diseasedPositiveNegativeTrue positive (a)False negative (c)False positive (b)True negative (d)a + bc + dTotala + cb + d From this table following measure can be obtained : -i) Sensitivityo It is the percentage of diseased people that wrho are diagnosed as having disease,o Total number of diseased people = True positive + False negative.o Number of people (among diseased people) diagnosed as having disease = True positives,o So, percentage of truely diagnosed people : -Number of true positiveSensitivity = ---------------------------------------------------------------------------------Number of true positives - Number of false negative.Specificityo It is the percentage of healthy people who are identified as healthy.o Total number of healthy people = True negatives + False positiveso Number of healthy people who are identified as healthy = True negatives,o So, percentage of true healthy people : -Number of true negativesSpecificity = ------------------------------------------------------------------------------------Number of true negative + number of false positives.Predictive valve of a positive test (positive predictive value of secreening test)o It is the percentage of truely diseased people among those who show positive test results,o Total number of people showing positive test results = True positive + False positiveo Numbers of truely diseased people showing positive test = true positive,o So, the percentage of truely diseased people : -True positivePositive predictive value = ------------------------------------------------------------------------True positive + False positiveiv) Predictive value of a negative of test (negative predictive value)o It is the percentage of healthy people among those who show negative test results,o Total number of people showing negative test results = True negative + False negativeo Number of healthy people showing negative test = True negativeo So, percentage of healthy people showing negative results.True negativeNegative predictive value = ---------------------------------------------------------------------------True negative +- False negative
True positive
False positive
True negative
False negative
Screening for Disease