Question

		Disease Prevalence 50%		Prevalence Disease 10%				Prevalence Disease 1%
		Yes	No		Yes	No	Yes		No
Test YES		TP   800 A	FP 100 B		TP   160 A	FP 180 B	TP 16   A		FP    198 B
Test NO		FN 200 C	TN 900 D		FN     40 C	TN 1,620 D	FN    4   C		TN 1,782 D
Totals		1,000	1,000		200	1,800	20		1,980
		NPV=   TN    OR   D    =           FN+TN      C + D X 100- ________%		NPV=				NPV=

Answer 1

TP - true positive means both test and disease is positive

TN - true negative means both test and disease is negative

FP - false positive means test is positive ,but disease is absent

FN - false negative means test is negative ,but disease is present

NPV is Negative predictive value . It is equal to true negative divided by sum of false negative and true negative

In disease prevalence 50% ,the TN = 900 ,FN =200

NPV = TN/FN+TN = 900/ 900+200 = 900/1100

= 0.81

In disease prevalence 10% , TN = 1620, FN = 40

NPV = TN/FN+TN = 1620+/1620+40

= 1620/ 1660

=0.97

In disease prevalence 1% ,TN = 1782, FN = 4

NPV = TN/ TN +FN = 1783/1782+4

= 1782/1786

= 0.99