EXAMPLE 12.1 Common Cause, Special Cause

Imagine yourself doing the same task repeatedly, say, folding an advertising flyer, stuffing it into an envelope, and sealing the envelope. The time to complete the task will vary a bit, and it is hard to point to any one reason for the variation. Your completion time shows only common cause variation.

Now you receive a text. You engage in a text conversation, and though you continue folding and stuffing while texting, your completion time rises beyond the level expected from common causes alone. Texting adds special cause variation to the common cause variation that is always present. The process has been disturbed and is no longer in its normal and stable state.

If you are paying temporary employees to fold and stuff advertising flyers, you avoid this special cause by requiring your employees to turn off their cell phones while they are working.