The positive predictive value is true-positive test results divided by all positive test results. This is also referred to as the predictive value of a positive test.
positive predictive value =
= (a / (a + b))
where:
• a = true positives
• (a + b ) = sum of (true positives and false positives) = all positive test results
If the prevalence of disease in a study is similar to that in the population [prevalence in the population = (a + c) / (a + b + c + d)], then the PPV is equivalent to Bayes's formula for post-test probability given a positive result.
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