Any data that is in measurement form, such as time to complete a task, can be converted to a capability index, provided at least one specification exists. So the recipe for a capability study is:
- Define what you will measure.
- Measure it consistently.
- Create a control chart such as an individuals and moving range chart from the data and ensure that the process is predictable. Most quality and patient safety departments already have done these three items.
- Determine the upper and/or lower specification limit.
- Calculate the capability values. Software can greatly aid in performing the calculations and determining the percent of data expected beyond your limit.
The math is not complex. Two common capability indices are typically computed: Ppk and Cpk. For more on these indices, and responses to frequently asked questions about capability, visit this article in our Quality Advisor.