Introducing the 5 Why “So What” Test

By Ron Pereira Updated on May 17th, 2017

We, over at Gemba Academy, are busy finishing up our 8 Step Practical Problem Solving course which is modeled after the Toyota Business Practice (TBP) methodology.

In one of the modules we’re exploring 5 Why Analysis in more detail since we’ve learned that many people think they know how 5 Why works… but often fail to come to the results they desire.

We’re teaching our students many “techniques” often left out of this important problem solving approach including something we call the “so what” test.   Some also refer to this as the “therefore” test.

Here’s how it works.

The Milk is Spoiled

Let’s say the “problem” we’re investigating is the fact the milk in the refrigerator has spoiled… a simple example most people have experienced at one time in their life (hopefully you smelled the milk before you drank it… but I digress).

So, the 5 Why analysis (5 is just a number…  sometimes you’ll need to ask more whys and sometimes you’ll need to ask less) might go like this.

1. Why did the milk spoil?

It was left in the fridge for too long.

2. Why was it left in the fridge for too long?

We didn’t drink it fast enough.

3. Why didn’t we drink it fast enough?

We had more milk cartons than we needed.

4. Why did we have more milk cartons than we needed?

We bought more milk cartons than we needed the last time we went shopping.

5. Why did we buy more milk cartons than we needed the last time we went shopping?

There was a sale on milk and we tried to save money.

Ask So

Once you are done with your 5 Why analysis there is still an important, yet often left out, step.

Namely, we must add the word “so” at the end of the each response while then working back to the top to make sure it all makes sense.  Let’s see how this works with our example.

There was a sale on milk and we tried to save money…


We bought more milk cartons than we needed the last time we went shopping…


We had more milk cartons than we needed…


We didn’t drink it fast enough…


It was left in the fridge for too long…


The milk spoiled.

Simple But Powerful

While this may seem like a simple exercise I promise you there will be times when the “cause and effect” relationship of your 5 Why analysis makes no sense after adding the word “so” to the end of the statement and working backwards.

When this is the case, making the necessary adjustments will make your 5 Why analysis far more accurate and powerful.

How Do You Do This?

Have you ever used this method before?  Or, do you have another method to check the cause and effect relationship of your 5 Why analysis?

  1. Penny Riordan

    January 11, 2010 - 3:56 pm

    The SO check is an excellent method of validating the jumps between the 5 Whys. However, a general comment about 5 Why Analysis. 5 WHy is great when the causal path is obvious. But I think it is viewed as being totally linear, and it’s not always the case. At each Because, there could be multiple potential andsers to the Why. We actual create a 5 WHY “tree” when we have multiple answers. Hope this makes sense.

  2. Ron Pereira

    January 11, 2010 - 4:09 pm

    Great comment, Penny… as it turns out we actually talk about this VERY thing (multiple paths) in this same 5 Why module since, as you stated, most cause and effect relationships aren’t perfectly linear.

  3. John

    January 15, 2010 - 1:01 pm

    I believe 5 x Why works for any kind of problems. We call the check method “back with therefore”. One idea how to avoid senseless 5 x Why: we need to consider at every WHY step what went wrong (= “actual” situation) from the standard (= “should” situation) in order to avoid asking why on excuses and normal things. It depends also if we would like to really learn something out of the problem or not, hopefully not looking for an excuse to deny the responsiblity.

  4. Confused about 5 why vs. Root Cause

    January 15, 2010 - 1:01 pm

    Are there any rules of thumb that would suggest that the root cause (as identified by the 5 why analysis) would fall into one of three categories: noncompliance to standard, no/inadequate standard, or no/inadequate system?


    January 18, 2010 - 12:33 pm

    Why did “we” think buying more milk would save money?
    Therein lies the root cause!

    Five-whys is a useful tool but like a hammer it should be used by a carpenter. Someone must be a skilled facilitator to be sure the line of questioning is logical and to know when to stop asking why.

  6. Matt Hahn

    January 18, 2010 - 1:05 pm

    There was a sale on milk so I am assuming they bought more thinking this would save money in the long run. Sounds similar to what many purchasing depts say as we scrap obsolete material!

  7. Darrell L. Smith

    January 28, 2010 - 11:34 am

    I have read some of the comments from the 5 why test. I am assuming they are using this as part of the fishbone anlaysis tool. The 5 whys facilitate trying to find potential root cause as you fill the “bones” of the diagram. The 5 whys are best used when the person who started the 5 why sequence is allowed to finish their line of reasoning. Then another person within the team framework takes a turn for another 5 why sequence. This is to continue until they team has exhausted all potential ideas on the problem. This is all part of a larger sequence of problem-solving technique where are tools are used in conjunction with the 5 why-fishbone process. The 5 why portion of the process is not intended to be used as a stand alone process.

  8. David

    January 28, 2010 - 11:45 am

    The five why analysis can be a great logical partner of another set of tools to eliminate problems; for an instance use a 5 why analysis if a fish bone or is/is not analysis was performed earlier and if the problem description is clear so we have enough data to do the analysis; with this we avoid to go too “early” to the final conclusions.

  9. Ron Pereira

    January 28, 2010 - 11:49 am

    Thanks for the comment, Darrell. I agree with you… for the most part.

    We do teach 5 Why as part of the overall problem solving process – including how to tie it into the Fishbone creation process… but there will be occasions when you experience a problem and simply begin to ask why it happened until a potential root cause is identified allowing you and your team to then implement a countermeasure (while then checking to make sure it worked of course).

  10. Terry

    August 20, 2021 - 9:46 am

    In my opinion, the 5-Why process should be taught as part of the understanding the problem or problem definition phase of a problem solving or improvement activity. It should help help identify and clarify what you initially think the problem is. Also, we should know that going thru the 5-Why process can often lead you to more than one potential contributor… and if you use the drill deep and drill wide concepts to this process you might lead to an even more potentially significant or important issue… In the spoiled milk example, you could drill down to a training issue… Why did we buy too much milk? because we didn’t have a plan or a budget because we were not adequately educated or trained to know the value of a budget.

    Then, if you drill wider, you might add to why the milk was spoiled, as a potential issue with the fridge temperature. Then you could expand to a fridge temperature being too high because of an improper set point, and inefficient fridge and down to an airflow issue due to a dirty air intake screen being too dirty or the heat exchanger fins being dirty…

    Then if you also look at what potential issue or action would lead to the most value, then you could find yourself working on multiple steps to help the original issue… In this case, maybe improved training and improved preventative maintenance…

    Just my thoughts.

    • Jon Miller

      August 20, 2021 - 4:06 pm

      Hi Terry
      Thanks for your comment. You’re right about the importance of asking why in order to revisit your understanding of the problem. However, the reason we avoid looking for causes at the very beginning of the process is because seeing a potential cause can result in thinking of solutions, or otherwise limiting our inquiry and understanding of the problem. In practice, the PDCA cycle is rarely a smooth one-to-one progression from P to D to C to A and back to P. Often there are loops within the Plan phase, when we think we understand the problem, look for causes, realize we didn’t understand the problem at the right level of depth or breadth, ask why, and refine our understanding.

Have something to say?

Leave your comment and let's talk!

Start your Lean & Six Sigma training today.