How to calculate positive predictive value? Background: Suppose an accurate diag
ID: 3204560 • Letter: H
Question
How to calculate positive predictive value?
Background: Suppose an accurate diagnositic test is available to detect a rare disease. Let D denote a person with the disease. Let T denote a positive result.
If a person has the disease, the conditional probability of a positive result is .99
If a person does not have the disease, the conditioned probability of a negative result is .99.
Using the information answer the following:
a) Assume a prevalence of .02. What is the positive predictive value?
b) Assume a prevalance of .01. What is the positive prdictive value?
Explanation / Answer
From information given, sensitivity of the test is: 0.99, and specificity of the test is: 0.99.
a. For a prevalence value of 0.02, the PPV is as follows:
PPV=(sensitivity*prevalence)/{sensitivity*prevalence+(1-specificity)*(1-prevalence)}
=(0.99*0.02)/{(0.99*0.02+(1-0.99)*(1-0.02)}
=0.6689 (ans)
b. PPV=(0.99*0.01)/{(0.99*0.01+(1-0.99)*(1-0.01)}
=0.5 (ans)
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