Creating an NP / P control chart

Plot a Shewhart control chart for the number/proportion of nonconforming units to determine if a process is in a state of statistical control.

  1. Select a cell in the dataset.
  2. On the Analyse-it ribbon tab, in the Statistical Analyses group, click Process Control, and then click the type of control chart:
    Option Description
    np Plot a Shewhart control chart of the number of nonconforming units.
    p Plot a Shewhart control chart of the proportion of nonconforming units.
    The analysis task pane opens.
  3. In the Process drop-down list, select the number of nonconforming units variable.
  4. In the Sample size group:
    • If the subgroup sizes are variable, select Sample size variable, and in the Size drop-down list, select the corresponding sample sizes variable.
    • If the subgroups are a constant size, select Sample size constant, and in the edit box, enter sample size.
  5. Optional: In the Id drop-down list, select the identifier variable.
  6. In the Control limits drop-down list:
    • To define control limits based on a multiple of the process sigma, select Sigma, and then in the edit box, type the multiple of the process sigma to define the control limits.
    • To define control limits based on the probability distribution, select Probability, and then in the edit box, type the probability that a single statistic exceeds the control limits, assuming the process is in control.
  7. Optional: To specify known process parameters, select the Known process parameters check box, and then in the Proportion edit box, type the known proportion of nonconforming units.
  8. Optional: To show the process mean and sigma statistics, select the Show process statistics check box.
  9. Optional: To show the center and control limit statistics, select the Show process control statistics check box.
  10. Click Calculate.