You misunderstand. A test that labels everyone positive would be 100% sensitivity but 0% specificity, and a test that labels everyone negative would be 100% spe

You misunderstand. A test that labels everyone positive would be 100% sensitivity but 0% specificity, and a test that labels everyone negative would be 100% specificity but 0% sensitivity. Deciding every case at once by a single coin flip would thus be useless. As the article says, "A perfect predictor would be described as 100% sensitive, meaning all sick individuals are correctly identified as sick, and 100% specific, meaning no healthy individuals are incorrectly identified as sick." Deciding every case at once with a single coin flip does not meet both critera of a perfect predictor.