Interpretation & Calculations
Process Center Line
When a Normal Distribution is assumed for the control chart statistic the centerline for the Individual-X chart is the midpoint between the calculated (or pre-defined, in that case) Upper Control Limit and Lower Control Limit. See Individual-X chart calculations.
When a non-normal distribution is assumed for the control chart statistic the centerline for the Individual-X chart is the median of the fitted distribution.
Learn more about the SPC principles and tools for process improvement in Statistical Process Control Demystified (2011, McGraw-Hill) by Paul Keller, in his online SPC Concepts short course (only $39), or his online SPC certification course ($350) or online Green Belt certification course ($499).