Poka-yoke and software development

Poka-yoke (the final “e” is pronounced like “eh?” in English) is the Japanese term for “error proofing”, formalized by industrial engineer Shiego Shingo as part of the Toyota Production System. (He is said to have picked the term “error proofing” rather than “fool proofing” [baka-yoke] to underscore that the problem was not foolish workers, but the fact that everyone makes mistakes from time to time when given a chance.)

In the Toyota Production System, poka-yoke deals with designing vehicles and their assembly processes in such a way that it is difficult to assemble them incorrectly.

Here is an example from the 1940s of the need for poka-yoke: My father worked at Kaiser-Frazer when they used to make a car called the Frazer. It had “FRAZER” spelled out in individual chrome letters on the front of the hood, like this.

Posts that stuck out the back of each letter went through holes in the hood to hold the letters in place. The positions of the posts were standardized, making all the letters interchangeable. Sometimes, when an assembly worker was having a bad day, he would end up making ZEFFERs instead of FRAZERs. Automakers subsequently changed the letters so they could not be interchanged, either by putting the posts in a different position on each letter, or by making the logo a single unit, like this.

A more modern example is the floppy disk drives that used to be in PCs. They used an unkeyed four-pin power connector that could be installed the right way, which would make the drive work, or the wrong way, which would make it go up in smoke. You had to remember that the red wire went away from the data connector, unless you were working with the odd brand where the red wire had to go toward the data connector. (I fried several floppy disk drives this way.) That was the last unkeyed connector I remember seeing on a PC, so the PC industry evidently adopted poka-yoke.

There are all kinds of modern examples, like polarized power plugs, and IKEA furniture, which is difficult to assemble wrong, and the same concept applies to software development.

For example, suppose you have a routine that performs several different functions. (That, in itself, is probably a violation of the Single Responsibility Principle, but suppose there is a good reason for it to be that way.) Some of the functions require few parameters, while others require many. Callers have to remember to pass the right number of parameters, including dummy parameters. So you get something like this:


If you miscount the commas, you have a problem. It would be much less error-prone to have multiple entry points, each with a fixed number of parameters.

The same concept applies to development tools. In one build system I worked with, you have to type in the names of all the modules that are part of your change. If the build fails because you left out one of the changed modules, you have to re-submit the build request, again typing in the names all the modules. If you have 30 or 40 modules in your change, you might mistype or leave out one of the names you got right in the first request, causing the build to fail again. If you could just call up the first request and say you wanted to add a module, there would be much less chance of error.

Another case is anywhere that you have to enter the same information into two different places. Eventually, you will forget to update one of them, or will mistype the information while entering it the second time. If the systems can be made to talk to each other, this greatly lessens the chance for error.

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