Sigma vs s.d. for Control Limits

I am seeing an increasing use of Tableau to create "control charts." Tableau recommends using s.d. to identify 'outlier' beyond the upper and lower 'control limits.' Without even talking about the lack of ability to use different types of charts or apply different tests...what's the practical effect of using s.d. vs. sigma for attribute data? I'm suspecting that we're missing special cause but can't find much written about this.

3 Replies
Duke Okes
235 Posts

Not sure there is a difference between s.d. and sigma for attribute data, since sigma comes from range chart which does not exist for attribute charts.

Jay Arthur
16 Posts

The only control chart that uses s.d. is the Levey Jennings chart which is most commonly used in laboratories. Attribute charts do not use standard deviation. Neither do variable charts. They all use an estimated sigma calculated from the data. You can look at the formulas here:

Why are there so many types of control charts? What makes them different? Learn more here:

Jay Arthur
16 Posts

If you want to learn more about control charts, I'll be doing a webinar on QI Macros - Lean Six Sigma SPC Software for Excel on May 10, 2022 9:00 AM MDT at:

I'll also cover key aspects of automating Lean Six Sigma to create improvement projects in minutes.