How to Create a P-Chart in Minitab 18
Why We Use P-Charts
A P-chart is one of the most commonly used types of statistical process control (SPC) charts. The P-chart tracks the percentages of attributes over a period of time. Whenever the process is going out of range, the person administering the P-chart can make adjustments when trends appear to be occurring. The number of samples in a P-chart can range anywhere from 2 to 30, but the sample size can be unlimited. The data used in the Minitab below is not from random sampling. Each sample had a size of 25. A sample was marked either satisfied or dissatisfied. The data important for the analysis would be the proportion of the samples where dissatisfaction was present. Download a copy of the Minitab file to follow along.
Add Data to Minitab
To add data to Minitab simply enter data into Minitab or paste data from excel. The data that I am mostly concerned with here is the frequency of dissatisfaction and the sample size.
Set Up the Pareto Chart
Now that we have data P-chart adjustment can be made. Start the P-chart generation process by clicking on Stat→Control Chart→Attribute Chart→P.
Add the "No. Dissatisfied" to the variables text box by double-clicking on "No. Dissatisfied" while the cursor is the in variables text box. Now add the sample size of 25 to the subgroup sizes.
Click on the "P Chart Options" button and click on the "Storage" tab to bring you to the storage options. Now click in the box labeled proportions. This will save the proportion of dissatisfied data on the worksheet within Minitab. Next, while still in the "Storage" tab, click on all the checkboxes in the below section except for the stage check box. This will allow these values to display at each data point. Next, click on the "Tests" tab and select "Perform all tests for special causes." Click Ok to exit P Chart Options.
Click on "Labels" to modify the labels on the chart. In the title section I want to add "P-chart Dissatisfaction" then click OK. Now click OK in the P-chart window to generate the chart.
At first, it appears that there is a problem with the P-chart because there are many points on the lower tolerance limit (LTL). This is actually a good thing. All of the points on the LTL represent samples where there was no dissatisfaction. The greater concern would be the high proportion of dissatisfaction trends in the chart. And when caught, corrections can be made.
Saving Your Project And Exporting The Graph
To save your file go to File →Save As, then select the location to save the project. There are plenty of ways to export the P-chart from the project. One way is to right-click on the graph and copy the image. Next, the image can be pasted where it needs to go. Another method is to right click on the image and export the image to a Microsoft Word or Excel document.
Boyer, K. & Verma, R. (2010). Operations & supply chain management for the 21st century. Mason, OH: South-Western.
© 2018 Joshua Crowder