5 Whys
Dig deeper to find the real cause
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What Is 5 Whys?
A simple iterative questioning technique used to explore the cause-and-effect relationships underlying a particular problem.
The 5 Whys is a questioning technique developed within Toyota as a critical component of problem-solving training. By repeatedly asking "Why?" (typically five times), you can peel away the layers of symptoms to reach the root cause of a problem.
The technique was originally developed by Sakichi Toyoda and was used within Toyota Motor Corporation during the evolution of its manufacturing methodologies. It is now used within Kaizen, lean manufacturing, and Six Sigma methodologies.
The key insight is that the first answer to "why did this happen?" is almost never the root cause—it's usually a symptom or an intermediate cause. By continuing to ask why, you move from surface-level symptoms to deeper systemic issues that, once addressed, prevent the problem from recurring.
- •Simple to moderately complex problems with clear cause-and-effect relationships
- •When you need a quick, conversational way to explore root causes
- •During team discussions to build shared understanding
- •When the problem has a relatively linear causal chain
- •As a starting point before more detailed root cause analysis
- •Highly complex problems with multiple interacting causes
- •When you need quantitative data to validate root causes
- •Problems requiring statistical analysis (use Pareto, control charts instead)
- •When team members have conflicting views that need data to resolve
- •Critical safety issues requiring formal investigation methods
- •Stopping at symptoms instead of digging to true root causes
- •Accepting vague answers like 'human error' or 'lack of training'
- •Not verifying each 'why' with evidence or observation
- •Pursuing only one branch when multiple causes exist
- •Using 5 Whys alone for complex problems requiring data analysis
Standard Example
Problem: Machine A stopped operating.
Why #1: Why did the machine stop?
→ Because the fuse blew due to an overload.
Why #2: Why was there an overload?
→ Because the bearing lubrication was inadequate.
Why #3: Why was the lubrication inadequate?
→ Because the lubrication pump was not working properly.
Why #4: Why was the pump not working properly?
→ Because the pump shaft was worn out.
Why #5: Why was the shaft worn out?
→ Because there was no strainer, and metal scrap got in.
Root Cause: No strainer on the lubrication pump intake.
Countermeasure: Install a strainer on the pump intake and establish a maintenance check for it.
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