Four basic categories of quality costs are described in the following:
1, Prevention----costs incurred in planning,implementing , and maintaining a quality system that will ensure conformance to quality requirements at economical levels. An example of prevention cost is training in the use of statistical process control.
2,Appraisal ---costs incurred in determining the degree of conformance to quality requirements. An example of appraisal cost is inspection.
3, Internal failure ---costs arising when products, components and materials fail to meet quality requirements prior to transfer of ownership to the customer. An example of internal failure cost is scrap.
4,External failure---costs incurred when products fail to meet quality requirements after transfer of ownership to the customer. An example of external failure cost is warranty claims.
Although the level of quality control is determined in large part by probability theory and statistical calculations, it is very important that the data collection processes on which these procedures depend be appropriate and accurate. The best statistical procedure is worthless if fed faulty data,and like machining processes,inspection data collection is itself a process with practical limits of accuracy,precision,resolution,and repeatability.
All inspection and/or measurement processes can be defined in terms of their accuracy and repeatability,just as a manufacturing process is evaluated for accuracy and repeatablity. Controlled experiments can be performed, and statistical measures of the results can be made to determine the performance of a method of inspection relative to the parts to be inspected. Suitability of one or another method can be judged on the basis of standard deviations and confidence levels that apply to each approach as used in a given inspection situation. (the end)
