Example of P Chart

The supervisor for a call center wants to evaluate the process for answering customer phone calls. The supervisor records the total number of incoming calls and the number of unanswered calls for 21 days. The supervisor creates a P chart to monitor the proportion of unanswered calls.

  1. Open the sample data, UnansweredCalls.MTW.
  2. Open the P Chart dialog box.
    • PC: STATISTICS > Control Charts > Attribute > P
    • Mac: Statistics > Attribute > P
  3. In Number of defective items column, enter Unanswered Calls.
  4. From the drop-down list, choose Subgroup sizes are in a column. In Subgroup sizes column, enter Total Calls.
  5. Click OK.

Interpret the results

The chart shows that, on average, 9.57% of calls are unanswered. None of the subgroup proportions are outside of the control limits. Furthermore, the points inside the limits display a random pattern. This P chart does not provide any evidence for lack of control. Thus, the process is in control.
Method
Parameter
Proportion
Proportion estimated
Tests for Special Causes
Results for P Chart
Test
Description
Failed Subgroups
1 point > 3 standard deviations from the center line
None
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