Statistical Process Control (SPC) Resource Center

Patterns of Variation

There are three common patterns of variation:


  • A bell-shaped curve.
  • This is a common pattern of variation.

Normal Distribution


  • Has two or more peaks.
  • A bimodal distribution signals that the process has a special cause of variation present. Something has changed in the process. There are operating differences between shifts, changes in processing methods, temperature and humidity differences, equipment differences, raw material changes, or other differences.

Bi-Modal distribution.


  • Looks like a normal curve pushed over to one side.
  • Skewed distributions also raise a red flag. But unlike bi-modal distributions, this non-normal distribution may just be showing the natural variation in the process if there is a natural upper or lower limit to the data. It could also show a problem in the process.

Skewed distribution.