# sensitivity/specificity problem

• Dec 17th 2012, 09:25 AM
LuluNeedshelp
sensitivity/specificity problem
A test has 60% sensitivity and 20% false positive rate. The test on a patient is negative. What is the probability that the test is a false negative?

How do I set up this problem to solve?

Thank you!
• Dec 17th 2012, 10:33 AM
HallsofIvy
Re: sensitivity/specificity problem
I think I understand "20% false negative" but what does "60% sensitivity" mean? That in 40% of the tests, it will not give a definite answer?
• Dec 17th 2012, 12:17 PM
LuluNeedshelp
Re: sensitivity/specificity problem
60% sensitivity= true positives/true positives + false negatives

20% false positive

I was thinking that if you have 20% false positive then you would have 80% true positive, so to solve for false negative:

sensitivity (60%)= true positive (80%)/true positive (80%) + X (false negatives)

I don't think I'm setting this up right because when I solve for X I get a negative number. So I either set it up wrong or I don't remember algebra or both :(