The other day I went to fill the gas tank on my car, like I have done thousands of times before. At the gas station I choose, you activate the pump by swiping your credit card, pushing a button to select the grade of gas, and then picking up the nozzle and inserting it into the gas tank.
In this case, after selecting the grade of gasoline I wanted, I went to put the nozzle in the tank, and it wouldn’t fit. I thought I had just missed the opening with careless placement, so I tried again. The nozzle still wouldn’t go in the tank. After the second attempt I realized that the nozzle was too big to fit into the filler tube on my gas tank.
Once I started actually looking at the nozzle, I realized that I had picked up the wrong one. This gas pump had separate hoses for gasoline and diesel fuels, and I had inadvertently picked up the diesel nozzle.
I thought this was a perfect example of a quality control technique called poka-yoke by the Japanese. Loosely translated, poka-yoke means mistake prevention, or mistake proofing. I usually call it idiot proofing. The idea is to design assemblies so that the parts can only go together one way. If they go together only one way, than the parts cannot be assembled incorrectly.
That is poka-yoke in its purest form. But the doctrine of mistake proofing has been used in many different ways. For example, in building an assembly, you can put quality checks in-line with the assembly process. Let’s say you put a switch on a base plate, then drive two screws to hold the switch in place. The next two stations in the assembly process could be a camera, to detect the presence of the two screws, followed by the installation of a cover plate to protect the switch. If you lock the assembly in place at the camera station, and only unlock it to move to the next step if the camera detects the screws, then you cannot put the cover plate on if the screws are missing.
Usually process controls like the one described above can be overridden by the people working on the line (usually a supervisor), which makes them less effective than going the route of designing an assembly that cannot be misassembled.
The problem with giving supervisors an override is that we assume mistakes are a function of poor training, or bad materials, or not caring about the job, or lack of experience. In my experience with the gas pump, however, the source of the error was none of those things. What happened to me was a momentary lapse of attention. So I was glad that the gas pump was designed so that I couldn’t get diesel fuel into the gasoline tank.
My experience with poka-yoke techniques at the gas station brought home another lesson for me.
No matter how much we idiot-proof our systems, we’re always outnumbered.
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