Sigma chart control limits are calculated as follows:
Control Limits calculated
from 
Control Limits calculated from 
is calculated using subgroup standard deviations instead of subgroup ranges. For more information, see Estimated Sigma. 

NOTE: If subgroup size is variable, m is replaced with m_{i} for i^{th} subgroup limits. When subgroup size is variable, the control limit values will not be the same; therefore, the chart may display `city skyline' type control lines.
Subgroup Sigma
Lower Control Limit (LCL) (optional)
Upper Control Limit (UCL) (optional)
Warning lines (optional)
Lower Control Limit (LCL)
Upper Control Limit (UCL)
Center Line (CL)