and s chart is a type of control chart used to monitor variables data when samples are collected at regular intervals from a business or industrial process.

However, Woodall[2] noted that "I believe that the use of control charts and other monitoring methods should be referred to as “statistical process monitoring,” not “statistical process control (SPC).”" The chart is advantageous in the following situations:[3] The "chart" actually consists of a pair of charts: One to monitor the process standard deviation and another to monitor the process mean, as is done with the

and s chart plots the mean value for the quality characteristic across all units in the sample,

, plus the standard deviation of the quality characteristic across all units in the sample as follows: The normal distribution is the basis for the charts and requires the following assumptions: The control limits for this chart type are:[4] As with the

chart is examined to determine if the sample mean is also in statistical control.

When samples collected from the process are of unequal sizes (arising from a mistake in collecting them, for example), there are two approaches: Effect of estimation of parameters plays a major role.

Therefore, several authors recommend using a single chart that can simultaneously monitor

and S.[8] McCracken, Chackrabori and Mukherjee [9] 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.