Prevention of mistakes and defects is a basic requirement of total quality and extends to every stage from supplier to final customer. It is far cheaper to prevent defects than to detect and fix them. Inspection that is done should be carried out on the process rather than on the product. Defects should be identified and rectified 'at source', not at some downstream process, inspection or rework station. This is the basis of Statistical Process Control (SPC).
SPC aims to create processes that are stable, predictable and capable. It involves sampling the process to make sure that no special events are occurring and that the process remains in control.
In addition, process capability studies make sure that the process is capable of easily holding the required tolerances. This is the basis of 'Six Sigma' which aims to create process chains having a minimum capability index of 2, reflecting capability limits at six standard deviations from the mean compared with normal process variations of three standard deviations.
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References:
Bicheno, J., Cause and Effect JIT: The Essentials of Lean Manufacturing, 2nd Edition, PICSIE Books, Buckingham, England, 1994 (ISBN 0 9513829 5 0)