Determining process capability

To determine process capability it is important that the process is in statistical control. It is meaningless to characterize a process that is unpredictable.

  1. Open the file Copper.xlsx.

    The worksheet is open and shows the data we used in the process control tutorial.

  2. On the worksheet, alongside the Phase column, click the drop-down arrow, then clear the IQ and Production check boxes, and select the OQ check-box, and click OK.

    The data is filtered to only the phase when the process was-in control is used.

  3. On the Analyse-it ribbon tab, in the Statistical Analyses group, click Capability, and then click 6-up.
    The analysis task pane opens.
  4. In the Process drop-down list, select Copper.
  5. In the Id drop-down list, select Date.
  6. In the LSL edit box, type 7.
  7. In the USL edit box, type 11.
  8. Select the Capability ratios and Nonconforming units check boxes.
  9. Click Calculate.
    The results are calculated and the analysis report opens.

The 6-up capability analysis shows the process control charts, a histogram of the distribution, a box-plot summary of the distribution, a mean +/- 3 sigma plot, and normal probability plot. Upper and lower specification limits show the specification.

Process capability indices(click to enlarge)
Based on the report you can see:
  • The process is in statistical control during the period.
  • The distribution is approximately Normal which is an assumption of most capability indices.
  • Ppk is 1.173 indicating the process is not capable meeting the specifications.
  • Pp is 1.414 indicating that if the process were centered between the specification limits it would just be acceptable.
  • It is expected the process will produce 216 non-conforming units per million.