Root Cause Analysis: How to Use the 5 Whys Method


November 19, 2020

How many times can you remember thinking you understood the cause of a problem, only to have it come back even after you “fixed” it?

Using the 5 Whys method can help avoid this type of situation by ensuring you dig deeper into problems—which is essential for developing solutions that reduce waste and cost.

What is the 5 Whys Method?

Developed as part of the Toyota Production System and prevalent in Lean methodologies today, the 5 Whys method involves asking ‘why’ repeatedly to uncover successive layers of a problem.

First, you look at a problem and ask why it happened. Once you answer the first question, you ask why that happened. Repeat this process five times in total and you will have typically found your true root cause.

The 5 Whys is one of the simplest root cause analysis tools available, and is useful for straightforward problems that don’t require advanced statistics. Let’s look at a detailed example of how to apply this method and what to do with findings to keep the problem from coming back.

Problem Statement

Let’s say scrap costs are significantly higher on one of your lines compared to the others. The first step is to write a problem statement.

Problem Statement: Machine A drops parts on the floor, instead of the conveyor belt 

Why#1: Why does the machine drop parts on the floor?

The first answer you need is why the part doesn’t reach the conveyor. It could be any number of situations:

  • The gripper on the arm may open too soon
  • The arm may not reach the proper position over the conveyor
  • The conveyor or arm’s base might be in the wrong position
  • The part may be slipping out of the gripper

 In this example, you find that the machine arm won’t rotate fully to reach the conveyor.

Why #2: Why won’t the machine arm reach the conveyor?

Just like in the first why, you have to look into why the arm won’t reach the conveyor, which could also be caused by any number of problems:

  • The gears could be broken
  • The arm could be twisted, crooked or bent
  • The base of the machine arm could be rotated out of its correct position

A critical step at this stage is to not assume, but dig in and answer the question specifically. In this example, you find that the scraper seal at the base of the arm’s rotational base is stopping the arm from rotating fully.

Why #3: Why is the scraper seal out of position?

In the base of the machine arm, a scraper seal has come loose and become bound up, preventing the arm from rotating fully. As with previous steps, the reason why could be any one of a long list of possibilities. In this example, you find the seal is too dry and got pushed out of position by the arm.

Now that you know that the seal isn’t properly lubricated, you may think you’re done. However, it would be a mistake to simply service the machine and move on. You need to keep asking why to get to a permanent solution that prevents recurrence.

Why #4: Why wasn’t the scraper seal lubricated?

You ask the line supervisor if the operator is supposed to lubricate the machine at any point. The supervisor confirms that the operator is supposed to lubricate the machine every 10,000 cycles and you see this reflected in his standard work.

Why #5: Why is the machine lubricated every 10,000 cycles?

You ask the process engineers who wrote the standard work about the lubrication step. Where did it come from? They tell you that it’s in the machine’s manual.

Inside the manual you learn that this machine needs to be lubricated every 1,000 cycles. Now you have identified the root cause of the problem: the machine has not been serviced frequently enough. In essence, a simple typo has led to a high scrap rate.

Acting on the 5 Whys

With root cause analysis complete, you can both solve the problem today and take steps to stop it from happening again.

At this point, you might take steps such as:

  • Having the process engineer that developed the standard work instructions change them to reflect the correct servicing of this machine
  • Adding the problem to a failure mode and effects analysis (FMEA) or a control plan to determine if it needs more attention
  • Creating an audit question based on the correction to verify the change is in place and working

This last step can be especially effective at bringing the issue to the team’s attention, particularly if you conduct layered process audits to verify key process inputs every day. An automated platform like EASE can help, allowing you to rotate the question in periodically to prevent recurrence.

The 5 Whys method has been around for a long time and is one of the simplest root cause analysis tools. You may reach the root cause in less than five steps, or it may take more, but using five as a guiding number ensures you dig deep enough into the problem to solve it once and for all.