The P chart consists of the following:

·    Plotted points, which represent the proportion of defectives.

·    Center line (green), which is the average proportion defective

·    Control limits (red), which are located 3 s above and below the center line and provide a visual means for when the process is out of control. The control limits are either fixed or varied, depending on your data and choices:

-   When your sample sizes are the same or when you choose to use an average sample size, then the control limits will be fixed.

-   When your sample sizes vary, then the control limits will vary. You should examine the P chart for points outside the control limits and trends or other nonrandom patterns.

Minitab conducts up to four tests for special causes for the P chart, which detect points beyond the control limits and specific patterns in the data. Points that fail are marked with a red asterisk and the number of the failed test. Complete results are available in the Session window. A failed point indicates that there is an unusual value or a nonrandom pattern, either of which may be the result of special-cause variation. These points should be investigated.

The P chart for the telephone data can be summarized as follows:

·    The lower and upper control limits are approximately 0.04 and 0.15, respectively. The proportions of defectives are expected to fall within the control limits. The center line (average proportion of defectives) is 0.0957.

·    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.