Reasons that the 5 Whys system may not always be successful include:
- Tendency for investigators to stop at symptoms rather than going on to lower-level root causes.
- Inability to go beyond the investigator's current knowledge, therefore they cannot find causes that they do not already know.
- Lack of support to help the investigator ask the right "why" questions.
- The use of the 5 Whys as a stand-alone tool, without using the preparation and preliminary phases mentioned.
- Investigation stopped at the 5th "why", even though more questions could/should be asked.
- Results may not always be repeatable – different people using the 5 Whys may come up with different causes for the same problem.
- Tendency to isolate a single root cause, whereas each question could elicit many different root causes.
- The output from the use of the 5 Whys tool can lack forms of verification to prove its correctness.
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