Cp and Cpk process capability poster

Process capability compares the spread of a stable process with its tolerance. Cp asks whether the spread could fit inside the tolerance; Cpk asks whether it does fit, where the process is centred now. This poster gives both in words, the rule that capability only means something for a stable process, and how to read Cpk using the acceptance levels in the AIAG PPAP manual: below 1.33 does not meet them, 1.33 to 1.67 may, above 1.67 does.

Download the poster

The PNG is free to download now. The print-ready vector PDFs open after a short form, once.

Download PNG (300 dpi)2550 x 3300 px, prints at 8.5 x 11 in · 477 KB · no form

Free to print and share with your team. Enter your name and work email once to download.

The words on the poster

Cp and Cpk

Could the process fit the tolerance, and does it fit where it is now?

Could it fit? Tolerance width ÷ process spread (6 sigma). Ignores where the process is centred.
Does it fit, here? Distance from the mean to the nearer limit ÷ 3 sigma. Off centre lowers it.
Stable first Check the control chart. An unstable process has no capability.

Reading Cpk

Above 1.67 Meets the PPAP level for a new process.
1.33 to 1.67 May be acceptable. Ask the customer.
1.00 to 1.33 Below the PPAP level. Reduce spread or centre it.
Below 1.00 Spread crosses a limit: out-of-spec parts. Contain.

Cp much higher than Cpk? The process is off centre. Re-centring is often the quickest gain.

1.33 and 1.67 from AIAG PPAP, 4th edition (2006). Bands below 1.33 are our guidance. Your customer decides.

On the floor

How to use it

Putting the Cp and Cpk process capability poster to work

1
Hang it in the quality office or the lab where capability studies are reviewed.
2
Check the control chart before quoting a Cpk: an unstable process has no capability.
3
Compare Cp and Cpk: a big gap means the process is off centre, which is often the easier fix.
4
Pair it with the free Cpk study sheet and PPAP checklist.

Where the poster goes and how to teach with it, in the order to try them.

Which size to print

24 x 36 in or A2: a main wall or team board, read from a few metres away.
18 x 24 in or A3: a station or notice board. The A3 file also fits 11 x 17 in paper.
Letter or A4: a clipboard, binder or handout, on any office printer.

Every PDF is vector, so it stays sharp at full size. Nothing runs to the edge of the page, so no bleed is needed.

Sources and notes

AIAG, Production Part Approval Process (PPAP), 4th edition (2006): acceptance criteria for initial process studies, index above 1.67 meets, 1.33 to 1.67 may be acceptable, below 1.33 does not meet. The poster's two bands below 1.33 (1.00 to 1.33, below 1.00) are its own guidance, not PPAP levels.
Douglas C. Montgomery, Introduction to Statistical Quality Control (Wiley): Cp = (USL minus LSL) ÷ 6 sigma, Cpk = the smaller of (USL minus mean) and (mean minus LSL), ÷ 3 sigma.