The 5 Whys is a root cause analysis technique used in the Analyze phase of Six Sigma DMAIC (Define, Measure, Analyze, Improve, Control). You state the problem, ask why it happened, then ask why again about each answer until you reach a cause you can actually fix. The goal of the 5 Whys is to get past the symptom to the root cause and stop the problem from coming back.

To understand why we use the 5 Whys, picture a child asking her dad “Why” repeatedly. It might drive him mad, but it teaches the child (and us) an important lesson in the Six Sigma Quality chapter.

5 Whys does not contain hypothesis testing, regression analysis, data segmentation, or other types of advanced statistical tools. Rather, it is a simple technique. Here, we ask “Why” until we can’t ask “Why” again. The number 5 is a good rule of thumb, but it is of course, arbitrary. You are free to ask “Why” as many times as needed. By doing so, we can quickly reach the underlying causes of a problem.

Origin

Sakichi Toyoda, one of the fathers of the Japanese industrial revolution and the founder of Toyota Industries Corporation, developed this technique in the 1930s. By the 1970s, this method became popular, and Toyota and many other firms still use it to solve problems.

How to Complete the 5 Whys

Step 1. Form a Team

Here the team members who gathered should be the ones familiar with the details, affected by the problem, or noticed the problem.

After this, select a master for the meeting who will lead the discussion, ask the 5 whys, and assign responsibility for the solutions the group comes up with.

Step 2. Define the Problem

After observing the problem, write down a clear problem statement that is acceptable to all team members.

Step 3. Ask Why Five Times

Answer at least five levels of “whys.” This seems to be easy, but sometimes it turns out to be the difficult part. The key part of this step is to start with the right question. The team can adapt two types of approaches in this step, such as single lane 5 Whys or multiple lane 5 Whys.

Single Lane Process

Multi-Lane Process

Step 4. Address the Root Cause and Assign Owners

After identifying the true root cause of the problem, the team needs to discuss and agree on the practical and the most effective countermeasures that fit their case to prevent the recurrence of the problem.

Step 5. Monitor the Countermeasures

Continue monitoring the countermeasures until the problem statement is eliminated effectively or completely. Repeat the steps of 5 Whys if necessary to ensure that you’ve identified the correct root cause.

5 Whys Example: Late Customer Shipments

Here is a simple 5 Whys example from a warehouse. The problem statement: orders shipped late to customers in the last two weeks of the quarter.

1. Why were orders late? Pickers could not find stock for many orders on the first try.
2. Why could they not find stock? The system showed items in bin locations that were empty.
3. Why were the bin records wrong? Replenishment moves were not scanned when stock was put away.
4. Why were put-away moves not scanned? Temporary staff hired for the quarter-end peak had no scanner login.
5. Why did they have no login? The onboarding checklist for temporary staff does not include system access.

The fifth answer is a process gap you can fix: add scanner access to the temporary staff checklist. Notice that the first answer (pickers are slow) would have led to the wrong fix, such as adding more pickers. Check the chain by reading it backward with “therefore.” If each step still makes sense, the logic holds.

Jefferson Memorial Example

Limitations of the 5 Whys

5 whys will not help us analyze the root cause if the cause is unknown. We cannot reach the countermeasure if the person or team doing this type of analysis doesn’t know the problem or provide meaningful answers to the questions. Sometimes assumptions that we make in each step might not be the real underlying cause of the problem–there may be more causes to the problem. Also, the 5 Whys are clearly unable to differentiate between causal factors and root causes, and the lack of rigor where users aren’t required to test for sufficiency of the root causes generated by the method. The success of this method completely depends, to a degree, upon the skill with which the method is applied.

5 Whys make use of the countermeasures approach rather than giving priority to a solution approach. In countermeasure, which is a robust method, a set of actions are taken to prevent the reappearance of the root cause problem. Still, in the case of the solution approach, we have to give priority to symptoms by finding out the real solution for the root cause rather than preventing it from recurring.

In industries such as nuclear, health care, aviation, and aerospace, the 5 Whys are inadequate for problem-solving and need more advanced root cause analysis techniques.

5 Whys vs Fishbone Diagram

The 5 Whys follows one chain of cause and effect. A fishbone (Ishikawa) diagram spreads out to list many possible causes across categories such as the 6Ms. Many teams use both. Brainstorm on the fishbone first, then run the 5 Whys on the two or three causes the data supports. For complex failures with several causes acting together, fault tree analysis is a better fit.

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