A false positive rate of 2 to 4 percent could be terrible if the real positive rate is much lower than that. Say 1 in 1000 people are actually carrying the sub

A false positive rate of 2 to 4 percent could be terrible if the real positive rate is much lower than that. Say 1 in 1000 people are actually carrying the substance you're looking for - a false positive rate of just 1% would mean that for every person you identify correctly, you'll falsely identify about 10 others.