I am trying to trying to have x bar chart and a s chart set up for constant sample size data. I was trying to get k=3 sigma limits. The option is show zones for x bar chart but the option is not allowed for s chart.
How can I get show zones/ k= 3 sigma limits for s chart?
The short answer is that the zones are applicable to xbar charts, but not to range or standard deviation charts.
The purpose of a control chart is to indicate whether a process is in or out of control. This is indicated primarily by the upper and lower control limits. In the special case of an xbar chart it is possible to apply additional rules based on the probability of certain patterns of data points (e.g. consecutive points above 2 sigma). These patterns are articulated as rules such as the Western Electric Rules, and relate to multiples of standard deviations. Drawing zones helps to identify and emphasis these patterns. Oh, and they look pretty too. But we can't add them just because they look pretty, they need to be meaningful too. Adding zones to a range or stdev chart would most likely result in people drawing incorrect conclusions from the chart.