Understanding Confusion Matrices

Eli Bendersky explains what it is a confusion matrix tells us:

Now comes our first batch of definitions.

  • True positive (TP): Positive test result matches reality — the person is actually sick and tested positive.
  • False positive (FP): Positive test result doesn’t match reality — the test is positive but the person is not actually sick.
  • True negative (TN): Negative test result matches reality — the person is not sick and tested negative.
  • False negative (FN): Negative test result doesn’t match reality — the test is negative but the person is actually sick.

Folks get confused with these often, so here’s a useful heuristic: positive vs. negative reflects the test outcome; true vs. false reflects whether the test got it right or got it wrong.

It’s a nice read.  The next step, after understanding these, is figuring out in which circumstances we want to weigh some of these measures more than others.

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