You can specify computations for control limits by entering a value for k (K Sigma), or by entering a probability for α (Alpha), or by retrieving a limits value from the process columns' properties or a previously created Limits Table. Limits Tables and the Get Limits button are discussed in the section Saving and Retrieving Limits in the Legacy Control Charts section. You must specify either K Sigma or Alpha. The default value for K Sigma is 3.
KSigma
The KSigma option enables specification of control limits in terms of a multiple of the sample standard error. KSigma specifies control limits at k sample standard errors above and below the expected value, which appears as the center line. To specify k, the number of sigmas, click the radio button for KSigma and enter a positive k value into the text box. The usual choice for k is 3, which is three standard deviations.
Alpha
The Alpha parameter option specifies control limits (also called probability limits) in terms of the probability α that a single subgroup statistic exceeds its control limits, assuming that the process is in control. To specify alpha, click the Alpha radio button and enter the desired probability. Reasonable choices for α are 0.01 or 0.001.