That's what a "false positive" is but Wikipedia also has a separate article on "false positive rate", which gives the formula FP / (FP + TN) Where FP is numbe

That's what a "false positive" is but Wikipedia also has a separate article on "false positive rate", which gives the formula FP / (FP + TN) Where FP is number of false positives, and TN is number of true negatives. So it's a third option: - Out of 1000 actually negative samples, 50 were tested as positive. So in the case of 1000 samples, 949 correctly testing as negative, 50 incorrectly testing as positive, and 1 correctly testing as positive, the false positive rate is 50 / 999.