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Field Book · Chapter 3 of 8 · 5 Why and fishbone, step D4

Find the Cause

Use a fishbone to list every possible cause, then a three-legged 5 Why for occurrence, detection and system, so the 8D finds why it happened and why it escaped.

By Chris Martin, quality manager. 2 minute read. Free, no signup.

Use a fishbone to list every possible cause before you fall for one, then a three-legged 5 Why that asks separate questions about occurrence, detection and the system. Most escapes are three failures, not one, and the fastest answer is usually the wrong one.

By Thursday the sort is running and the customer has stopped calling every hour. Now comes the part most plants rush: why did this happen?

The fastest answer is usually wrong. "Operator handled the parts roughly" closes a report, but it doesn't explain why 14 parts were scratched in the same place on the same face, all packed after the same date. This is D4 of the 8D, and it deserves more than a guess.

Start wide with the fishbone

A fishbone diagram lists every possible cause before you fall for one. Sort the ideas under the six usual headings: man, machine, method, material, measurement, and environment. Don't argue yet. Just list.

For the scratched housings, the team listed machining burrs, contact on the wash rack, handling at final inspection, the pack method, the totes, and damage in shipping. Then they checked each one against the evidence. Final inspection records were clean, so the scratches happened after inspection. That cut the list in half before lunch.

Then go deep with three legs

A plain 5 Why asks why the problem happened. A three-legged 5 Why asks three separate questions, because every escape is really three failures:

  1. Occurrence. Why did the defect happen?
  2. Detection. Why didn't we catch it?
  3. System. Why did the way we manage the plant allow it?

Here's how the three legs played out:

Occurrence. The housings were scratched because they rubbed against each other in transit. They rubbed because the totes had no dividers. The totes had no dividers because a new tote design arrived from the packaging supplier on September 28.

Detection. Final inspection happens before packing. Nothing checks the parts after they go into the tote, so damage from packing or shipping can't be seen by anyone in the plant.

System. The tote change was handled as a purchasing decision, not a process change. Nobody filed a change request, so nobody asked what the new tote would do to the part.

Now you have three causes, and each needs its own action. Fix only the first, and the next change to packaging, labels or carriers will slip through the same gap.

The mistake to avoid

Ending at "operator error" or "retrain." People make mistakes inside the system you give them. If retraining is your only action, ask one more why: what about the process made the mistake easy? Then fix that.

ON THE FLOOR MONDAY
  • Pull one closed corrective action.
  • Check whether it answers all three legs: occurrence, detection and system.
  • If it stops at "retrain operator," ask the next why with the team this week.
Want every tool on this trail in one purchase? The Complaint-to-Closure Trail Bundle has all ten, from the complaint log to the effectiveness tracker.

The plant, the part and the complaint in this book (RMA-031) are a made-up example. The tool sidebars show ShopFloorOps tools, which I make and sell. Every chapter teaches the step itself, so you can use your own forms.