A colleague whose company makes products for the aerospace industry wrote to me recently about a problem. Most items carried on airplanes or launched into space have a maximum weight specified, and often this is a critical parameter. In production, parts are weighed and a specification is applied. In some cases, when a part gains weight through the process (as components are added or treatments are applied), the part may be weighed several times before it's completed. At each weighing step, a digital scale is used, and a typical instrument will resolve perhaps 1/10,000 of a pound, or four digits past the decimal point. If the drawing specifies a maximum weight of 3.5 pounds (in exactly those words), what constitutes an appropriate scale reading for rejection? Various workers, inspectors and quality engineers in this firm would accept readings of 3.4999, 3.5000. 3.5499 and even 3.5999 pounds.
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