Root Cause Analysis And The Photocopier Question

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Root cause analysis, the "Five Whys", and the photocopier question

"Root Cause Analysis" is complex, involved, and requires both tenacity and creativity. When there's a problem, often the cause isn't a simple, single thing, it's often a collection of things, and each of those might not be easily characterised. Even so, it's a crucial exercise in many technology based industries and activities. So in this post I want to talk about my most recent brush with the challenges of performing RCA, and even of getting other people to see that it's there to be done.

The start of this episode

Recently I was reminded of a question I was asked quite some time ago, and which I found intriguing:

  • What do you get if you photocopy a mirror?

Now, some people just don't engage with puzzles and/or games. Some people insist on knowing how something will be useful and effective in their life. Some people want to know the utility of a thing, and if they can't see it, they'll move on to something whose usefulness is more obvious or overt. I'll call these people type P for Practical.

Others will engage with something simply because it's an interesting question. They are driven by the curiosity that a question like this can provoke. I'll call these people type C for Curious.

Neither of these points of view are necessarily right or wrong, and indeed, each has merits and each has faults. More than that, no one is entirely P nor entirely C, everyone is a mixture of both. And even then, the balance can shift, sometimes from moment to moment, sometimes from hour to hour, and certainly from month to month or year to year.

But it's definitely worth remembering that they both exist.

So this question of photocopying a mirror would seem just to be an idle speculation, albeit one that can be resolved by getting a mirror and performing the experiment. So it's often dismissed by those of type P as a pointless philosophical question.

Going beyond the simple problem statement

But I don't think it is. I think it is a great example of so-called "Root Cause Analysis." I'm not really just asking "What happens when you Photocopy a Mirror?" The answer to that question is like a symptom. I'm really asking: Why?

The "Five" in the "Five Whys" is a heuristic. Sometimes you can get to the root cause with just one or two, sometimes it takes more, and the method cannot be applied blindly and without understanding or thought.

In particular, sometimes there are multiple causes for a particular phenomenon, and sometimes different people will come to different conclusions. So the "Five Whys" is not hard, fast, and deterministic. It is at best a Guiding Heuristic.

So this is similar to the "Five Whys" method of root cause analysis. That has its merits and its faults, it's certainly not a universal panacea, but it is a useful concept to guide how we can proceed.

  • So what do you get when you photocopy a mirror?

    • You get result X (which will remain unrevealed in this post)

  • Why do you get result X ??

    • Because of the way the scanner and light source are arranged

At this point people stop. They think you need to know how the photocopier works at some level of detail that they don't care about, so the question is uninteresting and unenlightening.

But we continue:

  • Why are the light source and scanner arranged like that?

And here I get a blank stare. Who cares? Who knows? Who could possibly know?

But if you do the experiment not just on one copier from one manufacturer you find you get the same result from every copier from every manufacturer. There is something going on here - there is a root cause as to why this is the result.

My reasoning about this can be found elsewhere on this blog, but I'm not (yet) going to provide the link, because if you're interested, you should think about this first. If you get a different reason from mine, then we are both better off for having two ways of thinking about it. If you get the same reasoning that I came to, then that supports our conclusions.

If you give up, then you agree that it's non-trivial.

So why does this happen? We've moved beyond looking at the symptoms and we're asking about the reasons.

There must be a reason why we get the same result, and that will most likely stem from the task a copier accomplishes.

And that's the real question here. It's not "How does a copier work?" It's "Why must a copier produce the result it does when copying a mirror?"

It's not the "what," it's the "why".


There have been some comments and replies sent to me which I'm in the process of working through. That feedback will appear here in due course.


You can read about the "Five Whys" with a simple internet search. There are many many writings on the subject, and it wouldn't really be helpful for me to select just the one. So here's a search for "Root Cause Analysis" to get you started:

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