Continued Process Verification (Stage 3)

Process Capability Calculator

Calculate Cp, Cpk, Pp, and Ppk from your process mean, standard deviation, and specification limits — the core statistical tools used in Continued Process Verification (CPV).

Understanding Process Capability

Process capability indices quantify how well a process output fits within its specification limits. They are central to Stage 3 (Continued Process Verification) of the process validation lifecycle, and to ongoing statistical process control.

Cp vs. Pp — Potential Capability

Cp and Pp measure the spread of the process relative to the specification width, without accounting for centering. Cp uses within-subgroup variation (short-term); Pp uses overall variation (long-term, includes shifts and drifts between subgroups).

Cp = (USL − LSL) / (6 × σ_within) Pp = (USL − LSL) / (6 × σ_overall)

Cpk vs. Ppk — Actual Capability (Accounts for Centering)

Cpk and Ppk additionally account for how centered the process mean is between the specification limits — a process can have high Cp but poor Cpk if it is off-center.

Cpk = min[ (USL − mean) / (3 × σ), (mean − LSL) / (3 × σ) ] Ppk = same formula, using overall (long-term) standard deviation

Interpreting the Numbers

Index Value	Interpretation
< 1.00	Process not capable — output likely to exceed spec limits
1.00 – 1.33	Marginally capable — monitor closely, improvement recommended
1.33 – 1.67	Capable — commonly cited minimum target for validated processes
> 1.67	Highly capable process

Interactive Calculator

Enter your process mean, standard deviation, and specification limits. For a full CPV assessment, calculate both within-subgroup and overall standard deviation separately.

Specification Limits

Lower Specification Limit (LSL)

Upper Specification Limit (USL)

Process Data

Process Mean

Within-Subgroup Std. Dev. (σ_within)

Short-term variation, e.g., from moving range/R-bar

Overall Std. Dev. (σ_overall)

Long-term variation across all batches/subgroups

Cp

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Cpk

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Pp

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Ppk

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Using This in a CPV Program

Observation	CPV Action
Cpk close to Ppk	Process variation is stable batch-to-batch — good sign of control
Ppk notably lower than Cpk	Significant batch-to-batch shifts/drifts exist — investigate special causes
Cpk/Ppk trending downward over time	Early warning signal — trigger investigation before limits are breached
Cpk below 1.33 for a validated CQA	Consider process improvement, tighter control, or re-evaluation of control strategy

This tool provides a general statistical estimate for educational and monitoring purposes. Ensure your standard deviation calculations follow your site's approved statistical methodology (e.g., control chart constants for σ_within) before using capability indices for formal CPV reporting or regulatory submissions.