Free template
Process capability (Cpk) study sheet
A process capability study measures how well a stable process fits inside its specification limits. You take readings of one characteristic in small subgroups over time and compare the spread and the centering of the process with the lower and upper specification limits (LSL and USL). Cp = (USL − LSL) ÷ 6σ says how many times the process spread fits in the tolerance; Cpk = the smaller of (USL − μ) and (μ − LSL), divided by 3σ, also accounts for where the mean μ sits. Which σ you use matters: Cp and Cpk use the short-term σ within subgroups, usually R̄ ÷ d₂, while Pp and Ppk use the overall σ of every reading, which includes drift between subgroups. The PDF has a data sheet for 25 subgroups of 5 readings with sums, means and ranges, and a page with every formula, the control limits, a worked example and a histogram grid. The Excel version takes subgroups of 2 to 5, works out the x̄ and R control limits and flags subgroups outside them, and calculates Cp, Cpk, Pp and Ppk (one-sided when only one limit is given), with a histogram and a chart of the subgroup means.
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When to use it
When to use a process capability study
- For a new process or a new part, before production approval, often for the special characteristics in a PPAP.
- After a change to a machine, tool, material or method, to confirm the process still fits.
- To decide whether a characteristic can move from 100% inspection to sampling.
- When a customer asks for capability data on a characteristic.
How to fill it in
- 1
Check the gauge first
A gauge with poor repeatability adds its own spread to the result. Check it with a gauge R&R before the study.
- 2
Collect subgroups
Take 5 consecutive parts at regular intervals, 25 times: 125 readings. Record the time and anything unusual, such as a tool change or a new batch.
- 3
Work out means and ranges
For each subgroup, the mean (sum ÷ 5) and the range (largest − smallest). The Excel version does this as you type.
- 4
Check stability
Mean chart limits are x̿ ± A₂R̄, range chart upper limit D₄R̄. A subgroup outside the limits means a special cause: find it before quoting capability.
- 5
Calculate the indices
σ within = R̄ ÷ d₂ (2.326 for subgroups of 5); σ overall is the standard deviation of all readings. Then Cp, Cpk, Pp and Ppk.
- 6
Read the result
Compare with your customer's requirement. A low Cpk with a good Cp means the process is off center; a low Cp means the spread itself is too wide.
A filled-in example
Illustrative, not a benchmarkAn example: a shaft diameter of 10.00 ± 0.10 mm (illustrative numbers).
- LSL 9.90 mm, USL 10.10 mm. 25 subgroups of 5, no subgroup outside the control limits.
- Mean μ = 10.02 mm, σ within = 0.02 mm.
- Cp = 0.20 ÷ (6 × 0.02) = 1.67.
- Cpk = smaller of (10.10 − 10.02) and (10.02 − 9.90) = 0.08, divided by 3 × 0.02: 0.08 ÷ 0.06 = 1.33.
The spread would give a Cpk of 1.67 if centered, but the mean sits 0.02 mm high, so Cpk is 1.33. Moving the machine offset down to the nominal is the fix, not a new machine.
Common mistakes
Quoting Cpk for an unstable process
Capability describes a stable process. With points outside the control limits, the number does not predict anything.
Not saying which σ you used
Cpk from short-term σ and Ppk from overall σ give different numbers when the process drifts. Report both, labelled.
Too few readings
A Cpk from 20 parts has a wide margin of error. 25 subgroups of 4 or 5 is a common study size.
Ignoring the shape
The formulas assume roughly normal data. A histogram with two peaks or a long tail needs a look at the process first.
Download the template
Free to use: print it, copy it and edit it for your team. Enter your name and work email once to download.
Run this template in LeanSuite
In LeanSuite quality checks run as digital checklists with number limits, an out-of-spec answer can raise a tag or start a CAPA automatically, and KPI Builder tracks quality KPIs against targets.
In the lean glossary
FAQ
Process capability (Cpk) study sheet: common questions
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