Let U. equal the number of underestimates out of the last n forecasts. In other words,U is the number of error out of the last k that are positive.If the process is in control, the expected value of U is k/2, but sampling variability is involved ,so values close to k/2 would not be unusual. On the other hand, values that are not close to k/2 would indicate that the technique is producing biased forecasts