How to Create a P-Chart in Minitab 18

Updated on January 5, 2019
Joshua Crowder profile image

Joshua has work experience in manufacturing, distribution, and aerospace. He received his BBA in accounting from Kent State University.

P-Chart

A P-Chart is used to monitor the percentage of defective items in a process. Data in a P-Chart are classified by only two categories, like pass or fail. The P-chart is meant to monitor the stability of a process over time to correct instability.
A P-Chart is used to monitor the percentage of defective items in a process. Data in a P-Chart are classified by only two categories, like pass or fail. The P-chart is meant to monitor the stability of a process over time to correct instability.

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 to a process 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 this Minitab tutorial is not from random sampling. Each sample had a size of 25. A sample was marked either satisfied or dissatisfied. An important piece of data for the analysis would be the proportion of dissatisfaction present in the samples. Download a copy of the Minitab file to follow along.

Considerations for P-Chart Data

Before getting into an example take a look at some guidelines that should be followed to ensure that P-Chart results are valid. Below are guidelines for collecting data.

Data Must Be Ordered by Time

Control charts detect change overtime, so it is important that your data is in the correct order of time. The oldest data must be collected first and will appear at the top of the column if using columns or to right of the rows if using data that is set in rows.

Collect Data From Specific Time Intervals

Before the data is collected a specific time interval to collect that data needs to be established. It’s very important that an interval be as short as possible, so changes can be made to compensate for variations in the chart. Intervals could be every 10 minutes, every hour, every production shift, or every day. After the interval is decided on, the data collector must be consistent with that interval while collecting data.

Data Should Be Collected in Subgroups

Data should be collected in subgroups under the same conditions. Subgroups are samples of similar item in the same process that are under evaluation. If the subgroup data are not collected under the same conditions, distinguishing between special cause and common cause may be impossible.

The Subgroups Size Should Be Large Enough

To ensure that control limits are accurate even though they are estimated, the subgroup size for your P-Chart needs to be large enough. There is a formula to determine whether subgroups are large enough. The required subgroup size (n) depends on the mean proportion of defects (P-hat). The formula to determine whether subgroups are big enough is n=.5 / P-hat. The result of n will determine the least amount of a sample size you can have rounded to the nearest whole number. You won’t be able to calculate this until you know the proportion of defects in a process.

The Data Must Have Enough Subgroups

Your data must have enough subgroups to have accurate control limits. It is still possible to use a P-Chart on a preliminary basis if there are not enough subgroups. As more subgroups are collected the control limits can be recalculated.

Add Data to Minitab

Data can be pasted into Minitab easily from Excel and other applications. Select of the data that needs to be copied and press Ctrl +C. Next, open Minitab and paste the data in the upper left-hand corner of the table.

Pasting Data to Minitab

Note that there are other ways to transfer data into Minitab. Another simple way to get data into Minitab is by opening an Excel or text file within Minitab by choosing “file” then choose “open” to select a file.
Note that there are other ways to transfer data into Minitab. Another simple way to get data into Minitab is by opening an Excel or text file within Minitab by choosing “file” then choose “open” to select a file.

Set Up the P-Chart

Now that we have data P-chart adjustments can be made. Start the P-chart generation process by clicking on Stat→Control Chart→Attribute Chart→P.

Generate a P-chart

Enter Data

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.

P-Chart Options

Other Options

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.

Other P-Chart Options

Add Label

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.

Add P-Chart Labels

P-chart Analysis

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. When these trends are caught the appropriate corrections to the process can be made.

P-Chart

When using P-Charts we are dealing with nonconforming units in a sample. Since non-conforming units can’t be less than zero the lower control limit on a P-Chart must be zero.
When using P-Charts we are dealing with nonconforming units in a sample. Since non-conforming units can’t be less than zero the lower control limit on a P-Chart must be zero.

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.

References

Boyer, K. & Verma, R. (2010). Operations & supply chain management for the 21st century. Mason, OH: South-Western.

Minitab. (n.d.). Overview for P Chart. Retrieved January 5, 2019, from http://support.minitab.com/en-us/minitab-express/1/help-and-how-to/control-charts/how-to/attribute-control-charts/p-chart/before-you-start/overview/

Minitab. (n.d.). Overview for importing data. Retrieved January 5, 2019, from https://support.minitab.com/en-us/minitab/18/help-and-how-to/data-input-and-output/open-files-and-import-data/overview/

Minitab. (n.d.). Data considerations for P Chart. Retrieved January 5, 2019, from http://support.minitab.com/en-us/minitab-express/1/help-and-how-to/control-charts/how-to/attribute-control-charts/p-chart/before-you-start/data-considerations/

Questions & Answers

    © 2018 Joshua Crowder

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      • Joshua Crowder profile imageAUTHOR

        Joshua Crowder 

        11 months ago from SE United States

        Great! If it helped at least one person I'm satisfied.

      • Coffeequeeen profile image

        Louise Powles 

        11 months ago from Norfolk, England

        That was very helpful to read. Thankyou.

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