Process capability analysis with confidence intervals and non-normal handling Cp, Cpk, Pp, Ppk, Cpm, Z-benchmark, and sigma level — all with confidence intervals. Box-Cox and power transformations for non-normal data. Control charts in the same package to confirm stability first.

Capability indices that mean what they say

A capability index calculated from an unstable process is a meaningless number. An index calculated from non-normal data is a misleading number. Both mistakes are common, and both happen more easily when control charts and capability analysis live in separate tools — it becomes tempting to skip the stability check, or to run the indices without assessing the distribution first.

Analyse-it includes control charts and capability analysis in the same package, so the workflow stays in one workbook: confirm stability with a Shewhart chart, check the distribution with a Q-Q plot, transform if necessary, then calculate the indices. Capability indices (Cp, Cpk, Cpm) and performance indices (Pp, Ppk) are reported separately, all with confidence intervals. Z-benchmark, sigma level, and nonconforming unit estimation complete the picture for Six Sigma programmes.

Makes some aspects of my day to day work much easier than it used to be, where I either had to write my own software or use more cumbersome and expensive packages that had more functionality than I wanted and so were more effort to learn how to use.
Alastair H.
Chief Scientist
Medical Devices

What’s included

Measure inherent process capability

Capability indices Cp, Cpl, Cpu, Cpk, and Cpm — calculated from within-subgroup variation to reflect what the process can deliver when operating in control. A Cpk of 1.33 means the nearest specification limit is four standard deviations from the process mean; below 1.0 the process is not capable. All reported with confidence intervals.

Compare capability with actual delivered performance

Performance indices Pp, Ppl, Ppu, and Ppk — calculated from total process variation to reflect what the process has actually delivered over time. A process with good Cpk but poor Ppk has capability that isn’t being realised, which points to special-cause variation that needs investigation.

Quantify yield impact and sigma level

Z-benchmark (short-term and long-term) and sigma level calculated directly from the data. Nonconforming unit estimation — observed percentage, expected percentage, and expected parts per million — for both lower and upper specification limits, so you can quantify the practical impact on yield rather than just reporting an index.

Handle non-normal distributions correctly

Histogram with specification limits and normal Q-Q plot with Lilliefors confidence band included with every analysis. When the data is non-normal — common in real production — Box-Cox and other power transformations bring the distribution into a form where the indices remain valid. The Q-Q plot of the transformed data confirms the transformation worked before you report the result.

Confirm the process is stable before measuring capability

Shewhart variable and attribute control charts with WECO, Nelson, and Montgomery detection rules — in the same package. Run the control chart, confirm stability, then assess capability in the same workbook without transferring data to a separate tool.

Example analyses

See capability output in detail — histogram with specification limits, capability and performance indices with confidence intervals, Q-Q plot, and nonconforming unit estimation.

Process capability
Copper plating thickness with Xbar-R control chart. Xbar-R chart confirming stability, then capability analysis: histogram with specification limits, Q-Q plot, Shapiro-Wilk normality test, Pp, Ppk, expected PPM, Z-benchmark.

Part of the Quality Control & Improvement edition

Capability analysis is one part of the complete SPC and improvement toolkit. The Quality Control & Improvement edition also includes Shewhart, CUSUM, and EWMA control charts with detection rules, phases, and stratification, and Pareto analysis for identifying the vital few defect categories, plus the full Standard edition with hypothesis tests, ANOVA, and regression. See everything in the Quality Control & Improvement edition →

Validated, reliable, trusted for over 30 years

Validated calculations Every statistic tested against published datasets and thousands of internal test-cases. No reliance on Excel’s built-in functions — Analyse-it handles all calculations internally. See how we develop and validate Analyse-it →
Data stays on your PC No cloud processing, no uploads, no third-party access. Your data never leaves your computer — essential when working with proprietary process data, customer specifications, or regulated production records.
Standard Excel workbooks Charts and results are ordinary Excel workbooks. Share with colleagues, send to auditors or customers, archive for quality records — no Analyse-it licence required to view.
No formulas to break Results contain no formulas, so they can’t be accidentally edited or corrupted. The control chart you reported will be exactly what you find when you reopen the workbook.

Technical details

Capability indices

  • Cp, Cpl, Cpu, Cpk
  • Cpm
  • All with confidence intervals

Performance indices

  • Pp, Ppl, Ppu, Ppk
  • All with confidence intervals

Yield and sigma

  • Z-benchmark (short-term and long-term)
  • Sigma level
  • Nonconforming units — observed %, expected %, expected PPM

Distribution assessment

  • Histogram with specification limits
  • Normal Q-Q plot with Lilliefors confidence band
  • Box-Cox and other power transformations