Five Whys

Trace an observed failure to a testable process cause.

Procedure

• State an observable problem.
• Ask why and attach evidence to each causal link.
• Branch when several causes are plausible; stop guessing when evidence ends.
• Change the suspected process cause and check recurrence.

Questions

• What supports this causal link?
• Would fixing this cause prevent recurrence?

Evidence to collect

• Observed failure
• Evidence for each link
• Before-and-after measure

Limits

• Five is a prompt, not proof of causal depth; avoid ending with blame.

Sources

• ASQ, Five Whys and Five Hows: Main body

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