NCSS Statistical Software NCSS.comX-bar and s Charts Other Control Charts for the Mean and Variation of a Process The X-bar and s charts are very similar to the popular X-bar and R charts, the difference being that the standard deviation is estimated from the mean standard deviation in the former, and from the mean range in the latter. The X-bar and s charts are generally recommended over the X-bar and R charts when the subgroup sample size is moderately large (n> 10), or when the sample size is variable from subgroup to subgroup (Montgomery, 2013). Two additional control charts available for monitoring the process mean are the cumulative sum (CUSUM) and exponentially weighted moving average (EWMA) charts. The CUSUM and EWMA charts differ from the X-bar charts in that they take into account the information of previous means at each point rather than just the current mean. The CUSUM and EWMA charts are more sensitive to smaller shifts in the mean since they use the cumulative information of the sequence of means. However, CUSUM and EWMA methods also assume a reliableestimate or known value for the true standard deviation is available.