2014: Advent Quality: Precision Widgets

by on December 13, 2014

Just as you can’t test every widget on every criterion, you can’t test every criterion exactly.

The world is not exact. The world is not precise. If you specify that every widget must be exactly 2.21 mm wide, then almost every widget is going to fail – and those that pass aren’t going to be 2.21 mm across anyway.

Every specification defines a range of acceptable values, even the ones that pretend not to. If your specification says that the widget must be exactly 2.21 mm across, then what your specification means is that the widget must be close enough to 2.21 mm that your measuring equipment cannot tell the difference. Even if your measuring equipment is very precise, you’ll still be looking at some uncertainty. You might be able to measure something as 2.2100 mm, but it’ll still be 2.2100 mm plus or minus 0.00005 mm – which isn’t the same as 2.21 mm.

Sensible specifications, then, written by sensible people who intend to actually use their specifications, are written with ranges. Sometimes with more than one range – if you have a action limit defined more narrowly than your acceptance limits, then you have a chance to adjust your process before you make anything you can’t sell.

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