Statistical Process Control (SPC) Resource Center

Conducting a Process Capability Study

The steps for conducting a process capability study are:

  1. Preparing for the study.
  2. Determining the process output.
  3. Comparing the output to the spec.
  4. Taking action to improve the process.

A process capability study measures the capability of a specific piece of equipment or a process under specific operating conditions.

  • It is important to identify and record this information prior to the beginning of the process capability study.

Step 1: Preparing for the Study

To prepare for the study:

  • Define the processing conditions.
  • Select a representative operator.
  • Assure sufficient raw materials are available.
  • Make sure the measurement system is reliable.

Step 2: Determining the Process Output

To determine the process output, run the process and collect data as you would if you were setting up a control chart.

  • Make sure the process is stable using the same methods as for setting up a control chart.
  • Since common process capability calculations are based on a stable, normally distributed process, if the process is not stable, you should not conduct a process capability study.
  • Calculate the process mean and process variation for the measured output.

Step 3: Comparing Process Output to the Spec

A specification normally consists of the nominal, or ideal, measure for the product and the tolerance, which is the amount of variation acceptable to the customer. It is often referred to as “the spec.”

  • The distance between the upper spec limit (USL) and the lower spec limit (LSL) is called the total tolerance, or T.T.
  • The Cpk for a process is determined by calculating the Cpu and the Cpl. The Cpk is the lower of those two numbers.

Step 4: Taking Action to Improve the Process

  • There are a variety of activities that can be undertaken to improvement process such as 8D Problem Solving or Mistake-Proofing.