Definition
Capability indices compare process variability with the customer's tolerance limits. Cp says how many times the ±3σ width fits into the tolerance; Cpk additionally accounts for how well the process is centred. Cpk ≥ 1.33 is usually considered capable, 1.67 very good; a value below 1 means the process produces rejects even when running stably.
Quality control & SPC
Cp and Cpk use short-term (within-subgroup) variability and assume a process in statistical control; Pp and Ppk use total long-term variability including shifts between subgroups. The gap between Cpk and Ppk shows how much room there is in better process stability.
The indices assume a normal distribution — for skewed data (runout, roughness) use percentile variants or a transformation. Always complement them with a histogram with tolerances and an estimate of the nonconforming share (PPM), which production understands better than the index itself.
In Statistica
Statistics → Industrial Statistics & Six Sigma → Process Analysis computes Cp, Cpk, Cpm, Pp, Ppk, the PPM estimate and confidence intervals of the indices, including variants for non-normal distributions (Weibull, Johnson, percentile method), with a tolerance histogram and a normal probability plot. Capability can be monitored continuously alongside control charts.
Related terms
- Control charts (SPC)A control chart is the basic tool of statistical process control (SPC): it plots measurements over time with a centre…
- Standard deviation and varianceVariance is the average squared deviation of values from the mean; the standard deviation (SD) is its square root and has the same units as the data.
- Normal distributionThe normal (Gaussian) distribution is a symmetric bell-shaped distribution described by its mean and standard deviation.
- Six SigmaSix Sigma is a data- and statistics-driven methodology for process improvement.
- Measurement system analysis (Gauge R&R)Measurement system analysis (MSA) determines how much of the observed variability is caused by the measurement itself.
Knowledgebase guides
FAQ
- What is the difference between Cpk and Ppk?
- Cpk uses short-term within-subgroup variability, Ppk the total. A large gap means the process drifts between subgroups.
- Why is Cpk smaller than Cp?
- Because the process is not centred in the tolerance. Cp = Cpk only when the mean sits exactly in the middle of the tolerance band.
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Quality control & SPC
Updated: September 2026.