In order to build a control chart, we must have the values for the control limits. In the most cases, assuming normality of our data, we can simply use ± 3σ (3 sigma or 3 standard deviations) to calculate those limits which will tell us that 99.7% of our data lie within the control limits.
Sometimes, we do not have the standard deviation (σ) of our data beforehand, so we can calculate it by simple math. However, there are some cases that even that is not possible, thus we use factors that help to estimate them.
The control limits for the control charts presented before as well as the table with the factors to calculate them are below.