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with reinspecting casting lots (Table 1). Fatigue: Typically, subsequent


2


inspections are not planned events, and they tend to be performed on overtime or in addition to normal workloads. Beyond the stress associ- ated with getting the unexpected job done, fatigue often is a factor. Beyond physical fatigue, inspectors experi- ence visual fatigue and discomfort that leads to increased error rates. Both general and mental fatigue have been shown to harm visual inspection effectiveness, specifically by reducing inspector vigilance. A reduction in vigilance increases miss rates. Persistence of memory: Inspec- tors who are asked to review or sort parts they have inspected already have a memory of what they inspected in the past, including what defects were found and where, how they searched and how frequently defects were found. Generally, inspectors assume they missed something, so they tend to use a different search pattern the next time. Tis has the effect of reduc- ing the effectiveness of subsequent searches, particularly if the initial pattern was well planned. It has been demonstrated that inspectors tend to ignore items that have been previously judged “distracters” (non-relevant indications) and streamline the inspec- tion to only those areas with potential “targets.” Tis can increase the miss rate and reduce the false alarm rate. All-or-none effect: When, early in an inspection, inspectors find defects of certain types or in certain areas, they tend to fixate on these to the exclusion of seeing others. Research has shown if a particular defect is not spotted in the early going, it is not likely to be found in subsequent part inspec-


Procedure Trough further research


and feedback from metalcast- ers, Schorn found six influ- ences on inspectors tasked


“Repetitive sorting holds no guarantee for quality improvement.”


tions. Tis finding is more than simply persistence of memory; it is a recogni- tion that the training and behavior of an inspector is not going to be altered radically upon being presented with the same parts again. Defect density: In the prior


theoretical description of multiple inspection behavior, it was assumed the values of the miss rate or false alarm rate were unrelated to the frac- tion defective. Empirical data and experience confirm research showing that as the defect density becomes low, the miss and false alarm rates increase. Vigilance tends to decrease as the defect rate decreases, and in a second or third inspection, the rate of defects is lower than in the original lot. In addition, the inspector has an expectation of the defect rate of the lot based on prior experience. If no defects are being found as expected, inspectors begin to second-guess themselves. It is very difficult to erase their sense of what the defect rate ought to be. Finally, subsequent inspections tend to be performed more rapidly, with the reduced time on task reducing effectiveness. Motivational losses: Redoing a


job one has already completed can be demoralizing and defeating. Tis translates into a different attitude toward the inspection, whether or not the work was done by the same inspector or group of inspectors. Tis can translate into results that carry high miss rates and low false alarms. In addition, research has shown that boredom and the sense that the work is dreary and repetitive clearly impact inspection effective- ness negatively. Another aspect


is the inspector’s knowledge that findings in the second inspection are a request to “check their own work” and report their “mistakes” to management. Proper communication of the purpose and consequences of reinspection is vitally necessary to ensure additional inspections of the same lot by the same inspector uncover defects properly. Suboptimal conditions: Subse- quent inspections often are per- formed under different conditions from the initial, mass production situation. Tis is especially true where additional inspections are performed as a sort driven by the finding of a defect. Te sort might be performed in a remote location, impacting almost every aspect of inspection conditions, including lighting, ergonomics, layout, time, packaging and handling routines, and the availability of visual stan- dards and criteria. Behavioral pressures are associated with rework of this kind, under the scrutiny of others and at obvious increased cost. Te economics of payoff associated with inspector decision making is radically altered. While the customer is watching, “When in doubt, throw it out,” often rules in the absence of clarity of purpose and confidence in the location of the line between acceptable and unacceptable product. While effectiveness of inspection


is likely to be negatively impacted by the conditions surrounding mul- tiple inspections, the amount of this degradation is impossible to deter- mine precisely. It is possible to gain an overall sense of the direction of these changes.


Table 1. Summary of the Impact on Effectiveness of Multiple Inspections Factor


Fatigue ONLINE RESOURCE


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48 | MODERN CASTING January 2013


Persistence of memory All-or-none effect Defect density


Motivational losses Suboptimal conditions


Miss Rate Increase Increase Increase Increase Increase Increase


False Alarm Rate


No change to decrease Increase or decrease Decrease Increase


Increase or decrease Increase


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