An employee falls on the floor. Someone investigates. Why did they fall? They stepped on a wet spot.

That's a true answer. It's also the wrong place to stop.

Most 5 Whys investigations end here, or somewhere close. The first answer is true, plausible, and tactical -- so the team treats it as the root cause and designs a countermeasure. A wet floor sign. A new procedure for spotting spills. A safety briefing at next week's huddle. Six weeks later, someone else falls on the same floor.

Simon De Castro walked through this example in a KaiNexus webinar on DMAIC pitfalls. He's coached more than 300 belt projects to completion, and stopping too early is the single most common 5 Whys failure he sees. The problem isn't that the technique doesn't work. The problem is that practitioners use it on autopilot.

Push Past the First Plausible Answer

Watch what happens when you push further.

Why was the floor wet? A pipe was leaking.

Now your countermeasure changes -- you fix the pipe, clean the floor, maybe inspect other pipes. Better, but still shallow.

Why was the pipe leaking? It hadn't been maintained on schedule.

The countermeasure shifts again -- the conversation is now about training and accountability for maintenance staff. Better, but still pointed at people.

Why wasn't the pipe maintained? Because the maintenance program didn't include that area.

Now you have a root cause. Address the maintenance program, and you've prevented every fall in that area that would have followed from any pipe in any wing of the building leaking onto any floor. The first answer would have given you a sign for one location. The fourth answer gives you protection for the whole facility.

Two Questions to Verify You've Reached the Root Cause

Simon offers two questions to verify whether you've actually reached the root cause:

If this cause hadn't been present, would the situation have happened?

If you fix this cause, will the problem stop recurring?

Both answers need to be yes. If your candidate root cause fails either test, you haven't gone deep enough.

Check Your Chain by Reading It Backward

There's a second verification worth running, and it's one I hadn't seen until Simon's session: read the chain backward.

"Because the maintenance program didn't cover that area, the pipe wasn't maintained. Because the pipe wasn't maintained, it leaked. Because it leaked, the floor was wet. Because the floor was wet, the employee fell."

If reading the chain backward exposes a step that doesn't logically follow, you have a disconnected link in your reasoning. That's worth catching before you build an action plan on it.

Two More Mistakes Worth Watching For

Two other 5 Whys mistakes are worth naming.

The first is asking the question too loosely. "Why?" by itself produces drift. Different team members fill in different implicit objects of the question, and the chain wanders. "Why did the employee fall?" is specific. "Why was the floor wet?" is specific. "Why didn't this get caught earlier?" is a different question entirely and probably belongs in its own chain.

The second is treating the analysis as a single chain when the problem actually branches. The wet floor example splits naturally. The water on the floor has one causal chain (a maintenance problem). Whether the floor was cleaned before the fall is a separate chain (in Simon's example, a staffing model that allocates cleaning crews by company size rather than square footage, leaving certain high-square-footage areas underserved). Both branches are valid. Both have root causes. Pretending the problem is linear when it's actually branching means you'll fix one cause and watch the same incident recur for a different reason.

The deeper discipline isn't about the technique. It's about resisting the satisfaction of the first plausible answer. Teams under pressure want to close the analysis and move to action. Leaders under pressure want a quick win they can report. The first "why" usually delivers something that feels actionable, and the impulse is to take it and run. Doing so quietly guarantees that the same kind of problem will return, possibly in a different form, possibly to a different person, and the team will start over with a fresh 5 Whys that stops at the same shallow depth.

The fix is small. Before you accept a root cause, run both verification questions. Read the chain backward. Ask whether the problem is actually one chain or several. If you build that discipline into your improvement work, the 5 Whys becomes the powerful diagnostic it was designed to be, instead of a five-minute exercise that produces another wet floor sign.

For the full webinar on DMAIC pitfalls -- including fishbone diagrams that don't lead to action, the over-engaged sponsor problem, and why most action plans stall at training and warnings -- watch the recording on the KaiNexus webinar library.