The Investigator's Protocol

The "Big Four" Western Electric Patterns

You don't need a math degree or a spreadsheet full of formulas to use these principles. You simply need to plot the spots.

If calculating averages and standard deviations (SD) feels like a hurdle, let the StepChange Analyzer do the heavy lifting for you. Our tool automatically calculates your process average and defines the "normal" boundaries, highlighting these Western Electric patterns the moment they appear in your data.

When you look at your chart, keep an eye out for these signals defined by the Western Electric principles:

• Rule 1: The Outlier
  One single point falls far outside the "normal" bounds (3-sigma).
  Investigator's Action: Check for a specific, singular cause. Don't panic; it’s likely just a one-off event.
• Rule 2: The Run
  Nine or more points in a row stay on one side of the average line.
  Investigator's Action: The system has shifted. Something changed in the process—investigate what it was.
• Rule 3: The Trend
  Six or more points in a row are steadily climbing or falling.
  Investigator's Action: You have a "drift." Intervene now to steer the process back before it hits a danger zone.
• Rule 4: The Shift
  Two out of three consecutive points fall near the outer danger limits (2-sigma).
  Investigator's Action: This is an early warning siren. The process is being pushed to its limit—find the lever and stabilize it.