How to Calculate the Centerline of P Charts

Save

Control charts are used to determine stability and monitor changes to a process. A p-chart is a type of control chart used with attribute or categorical data, such as yes-no, win-lose or defect-no defect, according to the Quality Advisor website. Because the data is in the form of proportions, subgroup size can vary over collection periods. The centerline of a p-chart is the expected value of the proportions and is calculated based on sample data.

Things You'll Need

  • Calculator
  • Arrange data in two columns, with the first being the total number in the subgroup and the second being the total number of defects. If you were measuring number of defective toasters over the course of a year, the data may look like this:

    Toasters Made

    500

    400

    200

    200

    100

    90

    145

    256

    345

    321

    567

    Defects

    250

    267

    273

    266

    276

    220

    205

    296

    237

    265

    154

  • Sum the total of the subgroups. In this example, the total number of toasters made over the course of the year is 3,124.

  • Sum the total defects. In this example, the total number of defective toasters is 2,709.

  • Divide the total of the subgroup by the total defects to derive the centerline of the p-chart. In this example: 2,709/3,124 = 0.87.

References

  • Photo Credit Jupiterimages/Pixland/Getty Images
Promoted By Zergnet

Comments

Related Searches

Check It Out

How to Build and Grow a Salad Garden On Your Balcony

M
Is DIY in your DNA? Become part of our maker community.
Submit Your Work!