Capability Plot in Excel

One of Six Charts Created Using QI Macros Capability Suite

Create the Same Charts as Minitab's Capability Sixpack™ for a Fraction of the Cost

The Capability Plot is one of six charts created by QI Macros Capability Suite.

A capability plot is just another way to visualize the capability of a process. Below is an explanation of what the 3 lines represent.

Specification: 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.

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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