The reason why sensitivity and specificity are at loggerheads is shown in the slide. Where do you put the “cut off” at point A or B or C to discriminate the sick from the well when measuring a continuous measurement like blood pressure or blood sugar. At A, you’ll get virtually all the diseased (high sensitivity but include a lot of healthy people in the positive tests). At C, you’ll avoid including the healthy but miss half of the diseased people.