![]() Of TNs divided by the overall number of negative tests (if you get a Overall number of positive tests (if you get a positive test, how likely Value of a positive test (PVP) is the number of TPs divided by the Predictive value of positive and negative test: The predictive.Health people did the test call healthy?) Of TNs divided by the number of people w/o the disease (how many The specificity is the TN rate: the number People with the disease (how many sick people did the test pick up?) Is the true positive rate: the number of TPs divided by the number of Sensitivity and specificity (rate of TP and TN): The sensitivity.Total number of patients gives you the prevalence of the disease Prevalence of disease: The total number with UTI, divided by the.Number of TP, FP, FN, TN: These entries are in the cells of the table.This is a chart that you might make as part of the computerĪnd let's take a look at the two-by-two table. We run our test on a big group of people with and without urinary tract infections, and put the results into 2x2 tables like this: For now, though, let's assume that we've chosen a criterion (or one has been chosen for us), and that we simply judge people to be "high WBC" or "low WBC". The decision about where to set our criterion for calling someone healthy or sick based on their WBC is an interesting one that we'll come back to in a later session. The higher the WBC we require in order to call someone sick, the more we'll wrongly classify sick people as healthy, but the lower the WBC we require, the more we'll wrongly classify healthy people as sick. Whatever criterion we set for calling someone sick or healthy based on this test, there will be some people rightly classified as sick or healthy, and some people wrongly classified as sick or healthy. We might describe the situation graphically like this:Īs the WBC gets higher, the person is more likely to be sick, and as its lower, they're more likely to be healthy. ![]() Their white blood cell count seems higher than that of the average patient, but there's natural variation in this number, and it's not impossible that a healthy patient could have a WBC that high. Suppose that we want to know if a person has a urinary tract infection. Our topic this week is the properties of diagnostic tests, and their implications for interpreting test results. ![]()
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