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What is the 5 Whys Technique?

5 Whys is an iterative interrogative technique used to explore the cause-and-effect relationships underlying a problem for example the root cause of safety incidents. The goal is to determine the root cause of a problem by repeating the question “Why?”. Each answer forms the basis of the next question. The “5” in the name derives from an anecdotal observation on the number of iterations needed to resolve a problem. The technique was originally developed by Sakichi Toyoda and later used at Toyota during the evolution of its manufacturing methodologies. The technique is now used within Kaizen, lean manufacturing, and Six Sigma.


Here’s an example:


The Problem: The car will not start.
1st Why: The battery is dead.
2nd Why: The alternator is not working.
3rd Why: The alternator belt has broken.
4th Why: The alternator belt was well beyond its useful service life and was never replaced.
5th Why and the Root Cause: The car was not maintained according to the recommended service schedule.

5 Whys as a Root Cause Analysis Method for Workplace Incidents

The 5 Whys technique can also be used as a method for determining root causes of workplace incidents. What would 5 Whys look like in the context of a workplace incident investigation? Here’s the application of 5 Whys to an example mentioned in an OSHA fact sheet:


The Problem: A worker slips and falls, and suffers an injury.
1st Why: There was a puddle of oil on the plant floor.
2nd Why: Oil spilled from a compressor.
3rd Why: An oil leak from the compressor was not detected.
4th Why: The compressor was not inspected on a regular basis and repaired (if required).
5th Why and the Root Cause: The compressor was not in the maintenance system.

In theory it takes five “whys” to get to the root cause, but in practice there will be cases where you may use more or fewer than five “whys”.

Finally, according to the Washington State Department of Labor & Industries, these are the benefits of asking the 5 Whys:

  • Simplicity: Easy to use and requires no advanced mathematics or tools.
  • Effectiveness: Helps to quickly separate symptoms from causes and identify the root cause.
  • Comprehensiveness: Helps to determine relationships between various problem causes.
  • Flexibility: Works well alone and when combined with other methods.
  • Engaging: Fosters teamwork.
  • Inexpensive: A guided, team-focused exercise with no additional costs.

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