5 Whys

5 Whys is a root-cause analysis method that repeatedly asks why a problem occurred. It continues past obvious symptoms until the underlying cause is reached, so countermeasures can prevent recurrence.

Source checked1/1 values · 2026-10-25Methodology

Specification

1 source

Kind	methodology

Study notes

The specific number five is not the point; keep asking why until the root cause is reached and eliminated.
5 Whys is used with Gap from Standard problems, focusing on specific problem definition, goal setting, root cause analysis, countermeasure establishment, and checks, standards, and follow-up activities.
Root-cause analysis types listed are logic, one-variable-at-a-time (OVAT), and multivariate-at-a-time (MVAT).
Logic-based analysis such as 5 Whys requires no advanced math skills but requires critical thinking using inductive, deductive, and abductive logic.
Taiichi Ohno's example traces a machine stop from a blown fuse to an overload, insufficient lubrication, a lubrication pump issue, a worn pump shaft, and no strainer allowing metal scraps to enter.
Also called Five Whys.