Which formula expresses specificity?

Which formula expresses specificity?

• A. TN / (FN + TN)
• B. TP / (TP + FN)
• C. TN / (TN + FP)
• D. FP / (TN + FP)

Answer: (C)

Specificity measures how well a test identifies negatives. It answers: among those who truly do not have the condition, how many does the test correctly label as negative? This is the true negatives divided by all actual negatives, which is TN + FP. So specificity = TN / (TN + FP). That’s why the expression with true negatives in the numerator and the sum of true negatives and false positives in the denominator is the correct one. Using true positives in the numerator gives sensitivity (the true positive rate), while including false negatives in the denominator would mix in missed negatives, which isn’t part of specificity. FP in the numerator would describe the false positive rate, not specificity.

Specificity measures how well a test identifies negatives. It answers: among those who truly do not have the condition, how many does the test correctly label as negative? This is the true negatives divided by all actual negatives, which is TN + FP. So specificity = TN / (TN + FP). That’s why the expression with true negatives in the numerator and the sum of true negatives and false positives in the denominator is the correct one. Using true positives in the numerator gives sensitivity (the true positive rate), while including false negatives in the denominator would mix in missed negatives, which isn’t part of specificity. FP in the numerator would describe the false positive rate, not specificity.