Poka-yoke, or error-proofing

A few days ago, a friend of mine had a minor surgery. (He's recovering just fine.) One of the interesting parts of the preparation, though, was that the surgical team asked him to mark on himself where they were supposed to operate.

Wait, what? Isn't that something they should already know before they get this far?

This is a stock photo and not my friend.
But you get the idea. 

Well yes, sure. But think about it. One of the worst possible mistakes on the surgical table—short of losing the patient altogether, I mean—is the risk of operating on the wrong part of the body. And while it's very rare, nonetheless it has happened: you can google stories of people who went in to have their left leg operated on, and the surgeon operated on the right leg instead. The easiest way to prevent that kind of mistake is to ask the patient—who presumably knows where it hurts—to take a big blue marker and put a circle or an arrow on the spot that needs attention. It's easy to do, and it's just one more line of defense against error.

In fact, error-proofing (sometimes described using the Japanese term poka yoke, or ポカヨケ) is a fundamental Quality method. If you can make an error impossible (or very unlikely) then you save all the time and expense of having to fix it later. And of course in the case of a surgical error, "time and expense" doesn't begin to cover it. 

So how do you error-proof a process? There are almost as many answers as there are processes. But consider the process of cutting wooden rods or metal poles to a fixed length. In principle you could ask the cutter to measure each rod one at a time, and then cut it. This approach is slow, and it also introduces a lot of chance for error: every time the cutter measures a new pole, the measurement is plus-or-minus a certain amount. At the end of the shift, there's a good chance that no two poles are the same length. But you can error-proof the operation by giving your cutter a jig or fixture to seat the poles in. Then if he cuts across the top of the fixture, the poles will all be the same length.

You can go one step farther. Use a table saw with a rip fence. Then you can align every pole exactly the same. Depending on the exact nature of what you are cutting, you may even be able to cut several at once.

By extension, you can think of a lot of safety-guards on machinery as error-proofing devices. What else is a hand-guard, after all, than a device to prevent you from committing the error of reaching your hand inside the equipment while the gears are turning? The simplest hand-guard does exactly that. A more elaborate form is designed so that the machine cannot operate unless the guard is in place: this prevents you not only from reaching your hand into the gears, but from removing the hand-guard and trying to operate the machine without it. And overriding or defeating a machine's safety equipment is another kind of error.

One of the earliest works
describing double-entry
bookkeeping

In an entirely different vein, consider the invention of double-entry bookkeeping in the late Middle Ages. Double-entry bookkeeping requires that every transaction has to be entered into two different accounts. 

For example, if a business takes out a bank loan for $10,000, recording the transaction in the bank's books would require a DEBIT of $10,000 to an asset account called "Loan Receivable", as well as a CREDIT of $10,000 to an asset account called "Cash".*

Strictly speaking double-entry bookkeeping is an error-detection tool rather than an error-prevention tool, because the sum of all credits to all accounts must always equal the sum of all debits from all accounts. If these two sums ever fail to match, there is an error somewhere. But the difference between error-detection and error-prevention is more terminological than real. If you detect an error soon enough, you correct it before it has time to compound into larger errors farther along in the process. 

In cases where the cost of error is smaller, the methods for error-proofing are correspondingly less formal. Another friend works in retail. Sometimes customers call in saying, "I was there yesterday looking at the products against the far wall, and I want to buy the one on the left. Is it still there? Can you set it aside for me, and I'll pick it up later?" Of course nobody knows which one was "on the left" back when the customer was in the store, because products might have been rearranged since then. But it's easy enough for a clerk to walk over to the far wall, take a picture, and text it to the customer. "Is this the one you want? Great. It'll be waiting by the register when you come in." The potential risk in case of error is not as great as in surgery, but it's a simple way to double-check and it makes the customer happy.

And that, of course, is the point.

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* Quoted from Wikipedia, "Double-entry bookkeeping."