Control Charts for Variables

Two charts are used together: R-chart ("range chart") and X barchart ("average chart")

Both the process variability (measured by the R-chart) and the process average (measured by the X bar chart) must be in control before the process can be said to be in control.

Process variability must be in control before the X bar chart can be developed because a measure of process variability is required to determine the -chart control limits.

R-Chart for Process Variability:

UCL_R = D₄(R)

LCL_R = D₃(R)

where is the average of past R values, and D₃ and D₄ are constants based on the sample size

-Chart for Process Average:

UCLR = X bar + A₂(R)

LCL = X bar - A₂(R)

where X bar is the average of several past values, and A₂ is a constant based on the sample size