As the highest form of error proofing, source inspection for zero defects is such a powerful concept that it merits a close look.
Source inspection is 100% inspection, not statistical sampling, for causal factors that lead to defects. Defects are prevented through immediate action to remove causal factors. While this may sound complicated, it is not. In fact, examples of source inspection are everywhere around us.
The logic behind source inspection is pretty simple. Before completing an action of some kind, check for causal factors of defects. If causal factors are present, then the action is stopped before completion, and before the defect occurs. With the causal factor(s) removed, the action can be restarted and completed without creating a defect.
Everyday examples are all around us:
Automobiles. A little fender bender in a crowded parking lot is a defect that we can all do without. Possible causal factor: foot off of brake pedal. Source inspection: all vehicles with automatic transmissions have a safety interlock that requires the driver to have their foot on the brake pedal before shifting out of park.
On-line forms. In this case, the defect is missing information on an airline reservation, a catalog purchase, or whatever. Possible causal factors: error of omission or typographical error. Source inspection: if a required data entry field is empty, incomplete, or in the wrong format, the reservation or ordering process will not advance to the next screen until the field is filled correctly.
Thermocouples. A residential natural gas explosion would be a very bad defect. Possible causal factor: no pilot light. Source inspection: gas water heaters, furnaces, fireplaces, and so on have thermocouples. If the pilot light is on, the thermocouple is hot and gas is allowed to flow. If the pilot light goes out, the thermocouple cools down and shuts off the gas flow.
Does source inspection make statistical sampling and control charts obsolete? Absolutely not. Use statistical process control to reduce causal factor variability to minimize interruptions and increase productivity.