Specifications are often defined separately from the process that is being measured and without a great deal of consideration of how easily the process can meet them. This can result in either many failures and rejects or effectively redundant specifications, as the variation in the process fits badly or well within the specified limits.
Specification limits and process capability thus need to be considered together. The limits still cannot be too tight as if calibration is done under ideal conditions, process distribution may subsequently drift or spread, for example as a result of wear in a machine tool.