Question

A drug company is developing a new pregnancy-test kit for use on an outpatient basis. The company uses the pregnancy test on 100 women who are known to be pregnant for whom 95 test results are positive. The company uses test on 100 other women who are known to not be pregnant, of whom 99 test negatives.

The company anticipates that of the women who will use the pregnancy-test kit, 10% will actually be pregnant?

What is the PV- (Predictive value negative) of the test?

	A.	0.95 or 95%
	B.	0.90 or 90%
	C.	0.994 or 99.4%
	D.	0.5 or 50%

Answer 1

Predictive value negative =P(negative)

Let N=negative ,po - positive and P- pregnant.

As 10% are pregnant.

So, P(pregnant) = 0.1 and P ( not pregnant)=0.9

And as 95 out of 100 tested positive given that pregnant.

So, P( po | pregnant) =0.95 and P(N| pregnant) =0.05 , P(N|not pregnant) =0.99.

So, P(negative) = P(N|pregnant)*P(pregnant) +P(N|not pregnant) * P(not pregnant)

=0.05*0.1+0.99*0.9= 0.896 =0.90 approximately.

Hence option b) is correct i.e. 0.90 or 90%.