What is a P Chart?
The P chart plots the percentage of defectives in one subgroup as a data point. It considers the situation when the subgroup size of inspected units is not constant. The underlying distribution of the P-chart is binomial distribution.
P Chart Equations
- ni is the subgroup size for the ith subgroup
- k is the number of subgroups
- xi is the number of defects in the ith
Use Minitab to Plot a P Chart
Data File: “P” tab in “Sample Data.xlsx”
Steps to plot a P chart in Minitab:
- Click Stat → Control Charts → Attributes Charts → P.
- A new window named “P Chart” appears.
- Select “Fail” as the “Variables.”
- Select “N” as the “Subgroup Sizes.”
- Click the button “P ChartOptions” to open a window named “P Chart Options”.
- Click the tab “Tests.”
- Select the item “Perform all tests for special causes” in the dropdown menu.
- Click “OK” in the window “P Chart”
- Click “OK.”
- The P chart appears in the newly-generated window.
P Chart Diagnosis
Model summary: Since the sample sizes are not constant over time, the control limits are adjusted to different values accordingly. All the data points fall within the control limits and spread randomly around the mean. We conclude that the process is in control.