Estimating process capability

Plot a process capability histogram and compute indices to determine if a process can meet specifications.

This task creates a new process capability analysis report. You can also estimate the process capability directly from a process control analysis report using the Capability command in the Process Capability group on the Analyse-it ribbon tab.
  1. Select a cell in the dataset.
  2. On the Analyse-it ribbon tab, in the Statistical Analyses group, click Capability, and then click:
    Option Description
    3-up Compute capability ratios and plot a histogram and univariate plot.
    4-up Compute capability ratios and plot a histogram, univariate plot, and normal probability plot.
    6-up Compute capability ratios and plot a histogram, univariate plot, normal probability plot and control charts.
    The analysis task pane opens.
  3. In the Model drop-down menu:
    • If the data are individual observations, select Process variable .
    • If the data are rational subgroups, select Process variable with Subgroups.
  4. In the Process drop-down list, select the quality characteristic variable.
  5. If the data are collected in subgroups, in the Sample size group:
    • If the subgroups are identified by a variable, select Subgroup by identifier variable, and in the Id drop-down list, select the subgroup identifier variable.
    • If the subgroups are a constant size and formed by consecutive number of observations of the process variable, Subgroup size constant, and in the edit box, enter the number of consecutive observations per subgroup.
  6. Optional: If the data are individual observations, in the Id drop-down list, select an identifier variable
  7. In either or both the LSL and USL edit boxes, type the lower and upper specification limits.
  8. Optional: In the Target edit box, type the target value.
  9. Optional: If you are using control charts, in the Sigma estimator drop-down list, select:
    Option Description
    Long-term Use the overall standard deviation of the individual measurements.
    Short-term Use the within-subgroup sigma of the process (using the process control R-, S-, or MR- chart).
  10. Select the Capability ratios, Z benchmark, or Nonconforming units check boxes.
  11. Click Calculate.