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Page 43

INSPECTION AND TEST

23.43

FIGURE 23.7

Analysis of inspection errors.

Standard Sample Array. In this method, the inspector makes an “examination” by inspecting a prefabricated mixture of product consisting of good units plus various kinds of defects. The standard sample array is known by various names, including job sample. For added discussion, refer to Harris and Chaney (1969). All units were previously carefully graded by a team of experts and numbered for ready analysis of results. The inspector’s score and his or her pattern of errors all point to the need, if any, for further training or other remedial steps. (In effect, the check inspection is conducted before the inspection.) For example, in a company making glass bottles, an attempt was made to correlate process variables with the frequency and type of defects found by inspectors stationed at the cold end of the annealing lehr. The experiment failed because inspector variability from shift to shift exceeded product variability. This also threw suspicion on the accuracy of the inspection performed by the final product sorters at the end of the line. A standard sample array of 500 bottles was created and was used to examine the inspectors. (The examinations were conducted in the Training Department on a miniature lehr.) The suspicions turned out to be well founded (consulting experience of J. M. Juran).

Remedies for Technique Errors. The need is to provide the missing skill or know-how and to answer the inspector’s proper question, “What should I do differently from what I am doing now?” Unless the inspector is in a position to discover the answer for himself or herself, the answer must be provided by management. If no answer is provided, there will be no change in performance. The various methods of analysis discussed earlier all can provide some clues that suggest the type of remedial action needed. In particular, use can be made of the concept of finding the knack. Under this concept, the data on inspector performance are analyzed to discover which inspectors give consistently superior performances and which are inferior. Next, a study is made of the work methods used by both types of inspectors to identify the differences in methods. Analysis of these differences often discovers what is the secret know-how (knack) being used to get the superior performance (or what is the secret ignorance that results in poor performance). Finally, the knack can be transferred to all inspectors through retraining (Czaja and Drury 1981 and Cooper 1980) or through being embodied in the technology (Kusch 1979). Where the technique errors are the result of lack of job capacity, the foregoing may be of no avail, and the need may be to foolproof the operation (see below) or to reassign the inspector to a job for which he or she does have adequate job capacity.
Certification of Inspectors. In critical inspections involving inspector judgment (e.g., interpreting x-rays of critical welds), it is increasingly the practice to require that the inspector be formally certified as qualified to do this job. [See Gibson (1983) for a further discussion of inspector qualifications in offshore industries.] The qualification process follows a well-standardized series of steps: