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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.