Table 1. Model of a contingency (2x2) table.

	Negative test	Positive test
Diseased	FN	TP	Sensitivity, TP / D
Healthy	TN	FP	Specificity, TN / H
	PV(-), TN / NEG	PV(+), TP / POS
FN – false negative. TP – true positive. FP – false positive. TN – true negative. FN, TP, FP and TN are expressed as numbers. PV(-) - negative predictive value. PV(+) - positive predictive value. NEG – total number of negatives, i.e. NEG = FN + TN. POS - total number of positives, i.e. POS = TP + FP. D - total number of diseased, i.e. D = FN + TP. H - total number of healthy, i.e. H = TN + FP. Total population sample N = FN + TN + TP + FP.