To use a very simple example, if a forecaster is 100 percent confident that 40 percent of a given area will see measurable rain, the POP is 40 percent. Of course, 100 percent confidence almost never happens in science, so the formula is often more complicated. What if a forecaster is 50 percent sure that rain will occur and expects that, if it does occur, 80 percent of the area will get that rain? 50 percent of 80 percent is 40 percent, so the POP is 40 percent.