Creating an EWMA control chart

Plot an EWMA control chart 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 EWMA
    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 the Weight edit box, type the weight to be assigned to the current sample in the EWMA statistic.
  8. In the L edit box, type the multiple of the process sigma to define the control limits.
  9. Optional: To detect existing out-of-control situations quickly, select the Headstart check box, and in the a and f edit boxes, type the parameters.

    A value of a=0.3 and f=0.5 mimics a 50% headstart.

  10. Optional: To reset the value of the EWMA statistic after each out-of-control signal, select the Reset after out-of-control signal check box.
  11. Optional: To specify known process parameters, select the Known process parameters check box, and then in either or both the Mean and Standard deviation edit boxes, type the known value of the parameter.
  12. Optional: To show the process mean and sigma statistics, select the Show process statistics check box.
  13. Optional: To show the center and control limit statistics, select the Show process control statistics check box.
  14. Click Calculate.