By considering the use of a cumulative sum control chart, in the way described by Page [3] and Barnard [2], as the application of two sequential probability ratio tests to the observed series taken in reversed order, some approximate formulae for properties of this procedure are obtained. These are simple formulae relating the probabilities of detection of a given change in average value to the position and slope of the critical limits. It is suggested that these results may sometimes be useful as a guide to the limits to be used.