Five-by-Five Whys

Search around on the web for "root cause analysis", and you're likely going to find article after article discussing the Five Whys (or 5 Whys, or 5Y) technique. This method is especially popular in manufacturing, where the main concern is often productivity -- maximizing production rate and minimizing rejects. I've heard many Six Sigma and Lean practitioners talk about this as one of their favourite tools. I can understand why, too... it's easy to remember, simple to apply, and gets deeper than traditional problem solving. However, it also contains some traps.

For reference, here is one fairly detailed procedure for performing the 5 Whys: (link). In summary, you simply ask "why did this occur", and after you answer that, you ask "why did that occur", and so on. You keep going until you get to something fundamental, or until you reach something that's completely outside your control. A rule of thumb seems to be that 5 iterations is a reasonable average, but this is not a hard rule... it may take only 3 levels of "why", or you may still be asking "why" 5 weeks from now. It just depends on the problem.

Now, can you spot the problems in the procedure summarized above? I can think of a few, but there are two that I think are really important. First, there is the assumption that a single cause, at each level of "why", is sufficient to explain the effect in question. This is rarely the case! Most often, you need a set of jointly sufficient causes to create any single effect. Second, what if one of your "why" answers is wrong? Maybe your answer was possible, but what if the actual cause (i.e., set of causes) was something else entirely? Even worse, what if your seemingly plausible answer was completely out to lunch?

One of the advantages of the 5 Whys is that it gets you to fairly deep, underlying causes. A major disadvantage is that if you make a mistake answering just one "why" question, your entire analysis gets thrown off. Even worse, the earlier your mistake in the process, the further off your root cause is going to be. At that point, it won't matter if you ask "why" 5 times or five million times.

If you want to avoid these problems, try modifying the questioning process as follows. Once you've finished your initial line of questioning, go back to your answer for the first "why" and ask some other questions.

  1. What proof do I have that this cause exists? (Is it concrete? Is it measurable?)
  2. What proof do I have that this cause could lead to the stated effect? (Am I merely asserting causation?)
  3. What proof do I have that this cause actually contributed to the problem I'm looking at? (Even given that it exists and could lead to this problem, how do I know it wasn't actually something else?)
  4. Is anything else needed, along with this cause, for the stated effect to occur? (Is it self-sufficient? Is something needed to help it along?)
  5. Can anything else, besides this cause, lead to the stated effect? (Are there alternative explanations that fit better? What other risks are there?)

The point of these questions is to establish existence, necessity, and sufficiency. Keep asking these five questions for every cause, at every level of questioning. If you diagram all this, you will end up with a tree of causal factors leading up to the original problem. Some may be less important than others, but you will have a much more complete picture of the causes leading up to your problem. Even better, you may find a more important cause than previously considered. At the very least, you will have avoided the "straight-line causation" trap.

I hereby dub this modified process the Five-by-Five Whys (5x5 Whys). Feel free to share it with anyone you think might be interested.

Note: If you want to expand a Five-by-Five Whys analysis into something more robust, try upgrading to a Causal Factor Tree Analysis... it's the next logical step, and it's not much more work.


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