in statistical control, then the entire process is judged to be not in statistical control regardless of what the chart indicates. Unequal samples When samples collected from the process are of unequal sizes (arising from a mistake in collecting them, for example), there are two approaches: Limitations and improvements Effect of estimation of parameters plays a major role. Also a change in variance affects the performance of chart while a shift in mean affects the performance of the S chart. Therefore, several authors recommend using a single chart that can simultaneously monitor and S. McCracken, Chackrabori and Mukherjee developed one of the most modern and efficient approach for jointly monitoring the Gaussian process parameters, using a set of reference sample in absence of any knowledge of true process parameters. See also and R chart Shewhart individuals control chart Simultaneous monitoring of mean and variance of Gaussian Processes with estimated parameters (when standards are unknown) References Quality control tools Statistical charts and diagrams