Need to Create a Capability Plot But Don't Know How?

QI Macros can create capability plots for you right in Excel!

The capability plot is one of six charts created in QI Macros Capability Suite. Capability plots help you visualize process capability and determine if your data points fall within customer spec limits.

Capability plots contain three lines:

  • Specifications: The end points of the specification line are the Upper and Lower Spec Limits.
  • Within: equals the Standard Deviation of SUBGROUPS - (Rbar/d2 or Sbar/c4).
  • Overall: equals the Standard Deviation of ALL DATA.

How to Analyze a Capability Plot

Compare the Specification Line to either of the other two standard deviations

If the Within or Overall lines are wider than the specification line, it means you have data points falling outside of the Spec Limits and therefore your process is NOT capable.


capability plot example of process that is capable

Not Capable

example of capability plot for process that is not capable

Compare the Within and Overall lines to each other

The two lines should be about the same if the process is stable. Big differences mean an unstable process.

Capable and Stable

capable and stable capability plot

Capable, but NOT Stable


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QI Macros Draws These Charts Too!