Customer Requirements

MTBF and other wrong metrics. People love the MTBF because it is simple, a single number, and it feels like it represents ‘reliability’ (but it does not). Sometimes we try hard to convince people not to use it, but they still want to. AND … they still want to ask ‘will this work (or not)?’ Pick one. And, of course, those who specify the MTBF usually come back later, bemused, when something fails.

What is reliability then? Reliability is the probability that something will not fail within a particular duration under specified conditions. For example, we might be interested in the warranty reliability of a product. If the warranty period is 2 years, then the warranty reliability requirement presumes this is the particular duration. What if a product has an MTBF of 2.5 years … what fraction of products will fail in the warranty period? The answer lies somewhere between 0% and 77%.

What can you do? Try to work with people. If they use a crazy metric to describe required reliability performance … try and tell them what that actually means. Tell them that requiring an MTBF of 2.5 years because we want to limit failures during our 2-year warranty period … actually means anywhere between 0 and 77 % of products will fail during that warranty period … and if that doesn’t work, maybe it’s them and not you!

Customer Requirements

Customer Requirements

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