5 Whys is the simplest root cause analysis tool. Start with a problem, ask why it happened, take the answer, ask why that happened, and repeat until you can't go deeper.
Worked example
- Problem: The customer received the wrong item.
- Why? The picker grabbed SKU 7834 instead of 7843.
- Why? The bins are next to each other and look identical.
- Why? The warehouse layout puts fast-moving SKUs together regardless of visual similarity.
- Why? The bin assignment algorithm optimises for pick-walk distance, not error rate.
- Why? Error rate wasn't in the original design brief.
What to do with that
You've reached something changeable - the bin assignment algorithm. Now you can propose a fix: penalise visually similar SKUs being adjacent. Whereas "picker mistake, need more training" (the first-level answer) wouldn't have fixed anything.
Common failure mode
Stopping at "human error". If your 5 Whys ends with "someone screwed up", you haven't done the analysis. Every human error is enabled by a system condition. Keep asking.