Five whys, worked through one example

Five whys is the simplest root cause tool I know, and the one I reach for first. State the problem, ask why it happens, then ask why about the answer. Here's one made-up example worked all the way through.

The problem

Customer invoices go out late, usually 6 to 10 days after the work is done.

Asking why

1. Why are invoices late? They wait for the manager to approve them.
2. Why do they wait? The manager approves invoices only on Fridays, in one batch.
3. Why only in a batch? Each one has to be checked by hand against its purchase order.
4. Why by hand? About half the timesheets are missing the purchase order number.
5. Why is it missing? The timesheet form lets you leave that field blank.

Root cause: the form allows a blank purchase order number.
Countermeasure: make the field required, try it for a month, and count how many invoices go out within 2 days.

Notice the first answer, "the manager is slow," would have sent everyone in the wrong direction.

Check the chain

Read it backward with "therefore": the form allows a blank, therefore numbers go missing, therefore invoices get checked by hand, and so on up to the problem. If a step doesn't follow, you skipped one.

What I've learned using it

• Five is a rule of thumb. Stop when you reach something you can change, whether that's three whys or seven.
• Ask why about the process, never the person. "Because Sam forgot" is a dead end. "Because nothing reminds anyone" is something you can fix.
• If an answer splits into two causes, follow both. When there are a lot of branches, start with a fishbone diagram instead.
• It isn't fixed until you measure it again. Check the problem a few weeks after the change.

Five whys comes from Toyota. Sakichi Toyoda is usually credited with the idea, and Taiichi Ohno made it part of the Toyota Production System. The fishbone diagram is Kaoru Ishikawa's.