Short Term Capability Calculator

Track short term capability from measured subgroup variation. See indices, limits, and defect risk instantly. Use cleaner data to strengthen stable process control decisions.

Calculator Inputs

Used for range based within sigma estimation.
Required in summary mode or manual method.
Enter raw measurements for data driven capability analysis.
Use when raw measurements are unavailable.

Formula Used

  • Within sigma from subgroups: σwithin = R̄ / d2
  • Within sigma from moving range: σwithin = MR̄ / 1.128
  • Cp: (USL − LSL) / 6σwithin
  • CPU: (USL − x̄) / 3σwithin
  • CPL: (x̄ − LSL) / 3σwithin
  • Cpk: minimum of CPU and CPL
  • Cpm: (USL − LSL) / 6√(σwithin2 + (x̄ − T)2)
  • Z values: distance from mean to each limit in sigma units
  • Predicted PPM: normal tail probability × 1,000,000
  • Estimated yield: 100 − predicted defect percent

Short term capability assumes a stable process and uses within subgroup variation instead of long term overall variation.

How to Use This Calculator

  1. Enter the lower and upper specification limits. Add a target value if your process has a nominal center.
  2. Paste measurement values into the measurements field. Use spaces, commas, or new lines as separators.
  3. Choose a sigma method. Use subgroup range for rational subgroups, moving range for sequential values, or manual sigma for summary studies.
  4. Select subgroup size when using the range method. Only complete subgroups are used in the sigma estimate.
  5. If you do not have raw data, enter summary mean, sample count, and manual within sigma.
  6. Press Calculate Capability. The results appear below the header and above the form.
  7. Use the export buttons to save the calculated metrics as CSV or PDF.

Example Data Table

Subgroup Measurement 1 Measurement 2 Measurement 3 Measurement 4 Measurement 5 Mean Range
1 10.14 10.21 10.09 10.18 10.12 10.148 0.120
2 10.16 10.11 10.20 10.08 10.15 10.140 0.120
3 10.13 10.19 10.10 10.17 10.14 10.146 0.090
4 10.12 10.18 10.09 10.16 10.11 10.132 0.090

Example limits for this dataset can be LSL = 9.90, USL = 10.30, and target = 10.10.

FAQs

1. What does short term capability measure?

It measures how well a stable process can fit within specifications using within subgroup variation. It focuses on immediate process spread, not long term drift.

2. What is the difference between Cp and Cpk?

Cp measures potential capability from spread alone. Cpk also considers how centered the process mean is between the specification limits.

3. When should I use the subgroup range method?

Use it when measurements are collected in rational subgroups, such as parts sampled from the same machine setting or production interval.

4. When is the moving range method better?

Use moving range when data are time ordered singles and subgrouping is not practical. It estimates short term variation from consecutive differences.

5. Why can Cp be high while Cpk is low?

That happens when process spread is acceptable but the mean is shifted toward one specification limit. Centering needs improvement.

6. What does predicted PPM mean?

Predicted PPM estimates the expected number of nonconforming parts per million, assuming the process follows a normal distribution.

7. Should I trust capability indices for unstable processes?

No. Capability results are meaningful only after confirming process stability. Special causes, trends, or shifts can distort the interpretation.

8. What value of Cpk is usually considered good?

Many teams accept 1.33 as capable and 1.67 as excellent. Your actual requirement depends on risk, industry, and customer expectations.

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Important Note: All the Calculators listed in this site are for educational purpose only and we do not guarentee the accuracy of results. Please do consult with other sources as well.