If you are buying two or three complex assemblies per month from a contract manufacturer, it would be reasonable to check every one carefully; there’s a lot that could go wrong. However, if you are buying 100,000 simple subassemblies per month it makes no sense for you to 100-percent check them and probably isn’t practical anyway; life is too short, costs are too high.

So how do you inspect or test products when 100-percent checking is impractical?

My first suggestion is to get someone else to do it. Ask your supplier to do the checking and provide evidence to you that they have done so. Beyond that, you or your suppliers could use a sampling plan. This works on the basis that you inspect or test a defined number of samples from each delivered batch, and if more than a fixed number of these units pass then you accept the whole batch. If fewer than that number pass you reject the whole batch or, at least, require that the batch is entirely checked.

It sounds obvious, and you may already be doing it, but how do you choose the batch size and the pass/fail numbers to be statistically valid?

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