Mistake-Proofing

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Mistakes are NOT inevitable, but the logical consequences of remediable design. As such, it’s so much easier to avoid them than to correct them, especially if each one becomes a link in a chain of events that go off the rails as a result. If I’d continued in academia, perhaps eventually chairing a department, I’d buy as many copies of this book as there were members of my department — faculty, residents, nurse anesthetists, medical students. It’s slim (72 pages) and easy to understand — no formal process(es) to follow. Instead, the book provides several seemingly simplistic but very useful rules of thumb anyone can adopt. As Chase & Stewart write: “You don’t need a Ph.D. in statistics to apply it. In reality, mistake-proofing is more like a structured form of common sense.” For example: “The key to creating mistake-proofing devices and procedures is not to do too much at once. Instead, concentrate on clever, inexpensive methods to check for only one mistake at a time. If you have two possible mistakes, develop two separate devices or procedures to catch them.” Right on!

– Joseph Stirt

Mistake-Proofing: Designing Errors Out
Richard B. Chase & Douglas M. Stewart
2008, 72 pages
$16 – print
Available from Amazon

$10 – download
Available from Lulu

Sample Excerpts

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The best way to ensure the detection of a mistake is to make sure that something in the environment makes it very obvious that one has been made. A good example of an environmental cue is the inevitable “extra” parts that remain after a do-it-yourself repair project. These parts make it very clear that you have not reassembled the item correctly.

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Machine mistakes, being generally mechanical in nature, are better understand than human mistakes. They are, therefore, more predictable and easier to control. If we look closely at the different types of machine mistakes, we see that they fall into two categories: those mistakes we can see coming and those that catch us unaware.

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Employees experience a continuous stream of encounters – one defect is a low failure rate. Customers experience a single defect as a 100% failure rate.

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Toyota, which is very experienced at mistake-proofing, averages about twelve devices for each machine.

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Go/No-Go gauges are not limited to the shop floor. Customers often use such gauges to detect and prevent mistakes. Some amusement park rides require riders to be above a certain height (so they do not slip through the safety restraints) or below a certain height (to keep larger people off of rides meant only for small children). Parks do not want customers to discover they are too small or large after waiting in a potentially very long line. By placing a gauge at he front of the line, customers can tell if they are tall enough (or short enough) to go on the ride without waiting in line.

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Mistakes are random events and therefore we must continuously watch for them. Sampling is not good enough. It looks at only a small proportion of the outputs in a process.

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Most importantly, mistake-proofing is the only method we know that includes customers’ actions in the quality control system. The importance of this is emphasized by one study that estimates that customers of services are responsible for one-third of the problems they complain about.

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Remember that the goal is to develop clever, simple and inexpensive devices. Don’t immediately opt for the high-tech solution.

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