Many of the standard rules examine points based on Zones. The area between each control limit and the center line is divided into thirds. The third closest to the center line is referred to as Zone A, the next third is Zone B, and the third closest to the control limits is Zone C. Note that there are two of each of the Zones, one upper and one lower. See Control Limits on page 17 for more information.
Zone A is also referred to as the “3-sigma zone”, Zone B is the “2-sigma zone”, and Zone C is the “1-sigma zone”. These sigma zone terms are appropriate only when 3-sigma is used for the control limits, as it is in Statit Express QC.
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Sigma is the Greek letter for s and is used in this context to denote the spread of data.
Standard control limits are located 3 sigma away from the average or center line of the chart. These are called 3 sigma limits. The distance from the center line to the control limits can be divided into 3 equal parts of one sigma each.
Statistics tell us that in normal data dispersion, we can expect the following percentages of data to be included within the sigmas:
1 sigma – 68.3%
2 sigma – 95.5%
3 sigma – 99.7%
We can expect 99.7% of the process outcomes to be within the 3-sigma control limits if the data are normally distributed.