How many quality control inspection points to install in a system

Control points can be either built into the system or performed by inspectors.

The former requires an investment in prevention costs; the latter results in appraisal costs. Both are effective, but prevention is often more efficient because acceptable quality cannot be inspected into a product; it must be a part of the production process. Investments to prevent lost units may relate either to people or machines. Prevention costs and appraisal costs will be shortly defined .

In determining how many quality control inspection points (machine or human) to install, management must weigh the costs of having more inspections against the savings resulting from 

 

(1) not applying additional material, labor, and overhead to products that are already spoiled or defective (direct savings) and 

(2) the reduction or elimination of internal and external failure costs (indirect savings). Quality control points should always be placed before any bottlenecks in the production system so that the bottleneck resource is not used to process already defective/spoiled units.

Additionally, a process that generates a continuous defect/spoilage loss requires a quality control point at the end of production; otherwise, some defective/spoiled units would not be found and would be sent to customers, creating external failure costs.