A test with 90% sensitivity and 90% specificity sounds reliable. But use it for screening, where 1 in 100 people has the disease, and fewer than 1 in 10 positive results will be true positives. Our new Predictive Value Explorer shows why. Sensitivity and specificity describe how well a test detects disease in samples with a known diagnosis. Patients and doctors ask a different question: if the result is positive, how likely is it that I have the disease? The positive and negative predictive values (PPV and NPV) answer that question. They depend on a third number: the prevalence, or pre-test probability, of the disease in the people being tested. When the pre-test probability is low, as [...]