Avoiding the 5 WHYS Trap
Search around on the web for "root cause analysis",
and you're likely going to find article after article discussing
the Five Whys technique. This method is especially popular in
manufacturing, where the main concern is often productivity --
maximizing production rate and minimizing rejects. This method
is quickly becoming a favourite amongst investigators. This is
understandable, as it's easy to remember, simple to apply, and
gets deeper than traditional problem solving. However, it also
contains some traps.
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?
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.
- What proof do I have that this cause exists? (Is it concrete?
Is it measurable?)
- What proof do I have that this cause could lead to the stated
effect? (Am I merely asserting causation?)
- 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?)
- 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?)
- 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.
Source: Edited from an article written by Bill Wilson www.bill-wilson.net
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