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free120/Block 3/Question#26

A new test to detect the presence of malarial ...


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Specificity is how often your test is right about people without the disease:

= TN / (TN + FP)
= 95 / (95+5)
= 95%

drdoom  Put another way: Of all the people who are disease-free, how many (of those people) did you catch? In this case, there are 100 people who are disease-free; our test labeled 95 of those people as “disease-free”, but our test also called 5 of those people “positive”; so our test is good (sort of) but not that good. +  
drdoom  Also note how specificity only answers questions about people WITHOUT THE DISEASE; it only “deals with” people who are disease-free. (Sensitivity is the opposite: it is a formula which only deals with people WITH THE DISEASE. I think understanding that is better than coming up with some crazy Snout-Spout-Spin mnemonic, which I never remember anyway.) +