ABSTRACT
In managing variables the usual aim is not to achieve exactly the same length for every steel rod, the same diameter for every piston, the same weight for every tablet, sales figures exactly as forecast, but to reduce the variation of products and process parameters around a target value. No adjustment of a process is called for as long as there has been no identified change in its accuracy or precision. This means that, in controlling a process, it is necessary to establish first that it is in statistical control, and then to compare its centring and spread with the specified target value and specification tolerance.
