In this note we pay attention to a well-known problem not sufficiently analized with regard to probabilistic aspects in the statistical quality control. The main topic is the comparison between the control charts of the mean, under the assumption of two simple control limits, and a system of charts based upon added warnig limits. Under classical normality and independence assumptions with even sample size, the powers of the corresponding procedures are developped and compared w.r. to quite general alternatives. The main result is that the simpler method, i.e., the usual two control limut chart, is quite always preferred to a four limits scheme
