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International Symposium on Human Factors in Quality Control
International Symposium on Human Factors in Quality Control State University of New York, Buffalo, USA May, 1974 The problems of defining, analyzing and improving h u m a n capabilities in quality control have received considerable attention in the last two years. This symposium brought together experts on Human Factors in Quality Control from Europe and the USA who covered both industrial applications and theoretica~ research. The primary aJ a o~ the symposium was to review and collate the results which have been obtained in a variety of industries and research laboratories, but which have tmtfl now been published in widely diverse journals. A secondary aim was to outline future directions for research and attempt to co-ordinate this throughout Europe and the USA. Eighteen speakers made presentations during three days: the first day being devoted to industrial applications, the second day to theoretical models of human performance, and the third day to methods of improving inspection performance.
Applications in industry Dr Alan D. S w a n of Sandia Laboratories, Alburquerque, New Mexico, gave the first paper on industrial applications. He showed how Human Factors engineers needed to be deeply involved in quality control aspects during the development of new high technology products. He discussed a number of studies, from his own experience, dealing with respecter accuracy and showing that this was never as high as was expected by management. One of the most successful methods which have been tried in his studies of assembling and testing complex electronic products was to use the 'do-check-do' method, where two operators alternate between assembling part of an item and inspecting the other's work. While this required more highly trained operators, it produced considerable improvement in product quality.
Steel industry The inspection problems m the steel industry were covered by Dr Jan Moraal of the Institute for Perception, Soesterberg, Netherlands. He analyzed the inspection of steel
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sheets on a conveyor line by a number of different methods, ranging from an analysis of inspection reports filled in by the inspectors, through analysis of eye movements during on-line inspection, to analysis of errors in two laboratory simulations of the real task. All of these studies showed that a conveyor belt speed of 300 ft/min (91 m]min) was a critical point beyond which inspection performance suffered. Of considerable interest were his results on methods of training operators. He used two methods in a laboratory task, static training in which simulated sheets were presented statically to the inspectors, and dynamic training in which the sheets moved past the inspector with the same exposure time as in the static condition. Performance, measured by the signal detection theory index d', was found to be consistently better in the static condition than in the dynamic condition, with more improvement and higher overall performance. However, a second experiment in which half the subjects were deprived of a night's sleep before the testing showed that the static condition was much more drastically affected by sleep loss than the dynamic condition. The on-line inspection situation was improved by changing the workplace design, improving the lighting and reducing to four the number of fault categories into which inspectors had to sort.
Glass mspection G.J. Gillies of Pilkington Bros Ltd., St. Helens, UK, covered applications of Human Factors to inspection in the glass industry. His first application was inspection of moulded glassware for a long list of critical defects. The approach used here was to determine the optimum order of searching for faults, design a new lighting booth with specialized lighting for picking up each of the fault categories, and to institute a training method which increased in difficulty progressively with time both in the off-line situation and then in the on-line situation. A most important part of the implementation of the study was to involve both management and inspection personnel at all stages. In an extensive production trial with the new method, 15 inspectors missed less than 1% of the rejects which came to them. His second study was m inspection of sheet glass for bad cutting, consisting of either a corner missing
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on a rectangular sheet of glass or a wrongly shaped corner put o n t o the glass. This was investigated using a simulation of glass cutting faults marked o n a roll of clear cellulose acetate pulled across an overhead projector and the image focused onto a white screen on the floor. A range of conveyor speed was studied, showing that a significant fall in performance occurred at a conveyor speed of about 3000 m/h. This was equivalent to a plate transit time of just under 2:s.
R u b b e r components Inspection o f small rubber components by R.W. Astley of the Lucas Institute for Engineering Production, Birmingham, UK: showed that the inspectors were rejecting a high proportion of good items (average of 9.6%). An improved inspection scheme was introduced in which the main emphasis was on redesign of the equipment and the workplace. Simple methods study techniques allowed almost a doubling of performance speed with no change in quality of inspection. The optical environment of the inspection was improved by using a greater power magnifying glass, and small fluorescent tubes to produce a reasonable light intensity without excess heat. Coupled with this was a change from a white laminated bench to a green surface to decrease the high contrast of the black rubber components. Because of the stimulating social environment in which the inspectors worked, it was not surprising that the introduction of background music, which has been found to work successfully elsewhere, produced no great improvement in performance. Food processing A. Sinclair, Loughborough University of Technology, UK, described two applications of Human Factors techniques to the food processing industry. The first was examination of trays of moulded pastry for jam tarts where on-line observation had indicated that insufficient time was being allowed to the inspectors to achieve proper inspection performance. An off-line simulation using real trays of jam tarts investigated the difference between stationary and moving presentation of the trays at three different exposure times per tray, 22-5. 15 s and 7-5 s. An analysis of the results of the experiment in terms both of hit and false alarm rates and of single
detection theory parameters showed that performance improved with increased presentation time. However, with the increased presentation time there was more opportunity for inspectors to throw away good items and this was reflected in a slightly higher false alarm rate. At the speeds used in this experiment there was no difference between moving and stationary presentations, in accordance with theoretical work which showed that there is no difference in moving and static presentation up to an angular velocity of 8°/s at the eye. The recommendations of the study included doubling the number of inspectors, a proposal which increased the detection rate of faulty items by approximately 5% and more than paid for the additional inspection s t a l l The second application described by Sinclair was the inspection of chicken carcasses in a chicken-packing factory. The job entailed taking from the the moving conveyor belt any carcass which was substandard because of bruising, broken limbs, etc. It was found that inspection standards were not rigidly defined but were determined to some extent by the state of the market. An experiment was designed with the conveyor moving at different speeds past inspectors on line at the same time of day, and this showed again that the percentage of rejects correctly identified improved with increasing presentation time up to approximately 1 s per bird. However, an anomalous finding was noted that above about 1¼ s per bird the hit rate decreased again. It was also noted in this experiment that performance steadily improved over the first 25 min of the task and then began to deteriorate in the manner expected with classical vigilance studies. To achieve good performance on the inspection task it was recommended that inspectors should be changed every 40 min during the day, and that the conveyor speed should be reduced wherever possible to allow at least 1 s per bird inspected.
Human performance models In the second day of the conference, three models of human performance apphcable to inspection were discussed, each model being presented and followed by a case study. Vigilance Classically, inspection has been regarded as a vigilance task so the first paper here was a presentation by Angus Craig, Medical Research Council Applied Psychology Unit, University of Sussex, UK, reviewing human performance models in vigilance, it was concluded that no unique theory had
emerged which would explain the experimental results found in studies of vigilance. The classical drop in performance with increased time on the job (the vigilance decrement) could either be explained as the inspectors' reduced expectancy for finding defects or signals or, on the other hand, by his reduced level of arousal. Despite this, many practical factors affecting vigilance performance have been found in laboratory studies. These include the effect of environmental conditions, such as ambient temperature or noise level, or of the time of day at which the inspection is performed, there being a larger drop in performance when inspectors are working during the night. Personality variables such as introversion/extraversion also appear to affect the results, with extraverts appearing to benefit from increased social interaction during the task. The major task related variables include the a priori probability that an item passing the inspector is defective: if an increased probability of defective occurs then performance improves. This has led to a number of recommendations that 'artificial signals' could be introduced into the inspection task to improve the arousal level of the inspector and, hence, his inspection performance. "t was pointed out by Dr Craig, however, that ff artificial signals were used, then full feedback of results had to be given Partial reinforcement by feedback produced a drop in performance when the feedback was removed.
Background music Recent studies have shown that a considerable proportion of the vigilance decrement observed in practice is due more to a shift in the subject's criterion than a change in his ability to discriminate good from bad product per se. As a case study in vigilance, J.G. Fox of the Ergonomics Information Analysis Centre, University of Birmingham, UK, discussed a study in a manufacturing company making small metal fasteners. Batches of metal fasteners were visually inspected to remove three types of defect. They were inspected under three conditions; first, normal working conditions with an ambient background noise of 83 dB; secondly, normal working conditions with a background of vocal music played for 10 min every 30 min; and thirdly, in a quiet condition where the background noise was reduced to 60dB(A). The results showed that there was a 7" 1% increase in fault detection under conditions of music. This was interpreted as being due to the increased arousal level of the inspectors
in the music condition over and above the other two conditions. In a second study in another factory, when silent conditions were compared with either unselected music or lively music, there was a progressive increase in fault detection with the music condition and accompanying this a reduction in the performance decrement in the second half-hour of work. Again, this confirms that arousal theory may be one of the keys to maintaining inspection performance. In the discussion of this study it was noted that if the music was changed to reflect an individual's personal preferences the fault detection scores were higher. However, it was suggested that if a resident disc jockey were employed to answer requests for music this might well be more distracting and meaningful than the task of inspection and lead to a decrement in performance. Visual search The theory of visual search, as presented by John R. Bloomfield of Honeywell Inc, USA, describes the situation in which an inspector must search either a regular or an irregular array of products, or even large sheet products, in order to locate and decide about individual defects. Bloomfield's review covered models of searching strategy ranging from completely systematic scanning, through systematic scanning with imperfect fault detection, to completely random scanning. It appears that most subjects" have a particular search strategy but may not follow this very exactly from trial to trial. All the models predict that as more time is allowed for the search there will be a higher probability of finding a defect, but the curves relating probability to time found in practice are reasonably insensitive to the model used. In an attempt to produce a predictive model as well as a descriptive model, Bloomfield's own work on search was discussed. This model describes accurately the search times obtained in looking for deviant items among an array of similar items or looking for small items on blank fields. It was deduced from the theoretical work that for good inspection one should choose inspectors with good peripheral acuity and that it may be possible to train inspectors to scan display material more efficiently. It was also shown that the search strategy will depend on the particular fault being looked for, so that if a number of faults need to be detected in the same area there is probably no single most efficient search strategy for them all.
The case study in visual search was also provided by Bloomfield. He took
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this opportunity to attempt a classification of the literature in visual search and inspection to produce the first step in a taxonomy of inspection tasks. This was considered by most people in the conference to be a most essential step in understanding and predicting performance in inspection. The fact that particular theories only apply to small parts of the task and that conflicting findings occur on different inspection tasks is probably due to the fact that the different inspection tasks emphasize different sorts of behaviour on the part of the inspector. It was emphasized that breaking down an inspection task using such a t a x o n o m y would have great advantages in field work on inspection.
State University, continued the exposition of signal detection theory in his case study paper. He showed how this theoretical framework could be used to study the inspection of small pieces of wire for cuts, the inspection of micro-miniature Landhott rings on a binocular microscope, and to elucidate visual search for letters. He introduced the concept that reaching a correct decision of accepting or rejecting an item involved first reaching a correct decision on whether or not to observe the item at all. There is no reason why both this decision and the decision on quality once observation is made cannot both be treated by signal detection theory.
Decision making
I nsl~etion performanceimprovement On the final day, a number of different approaches to improving inspection performance were covered.
Human decision making in quality control was reviewed by C.G. Drury of the State University of New York at Buffalo. Recent research in sampling quality control (SQC) plans had shown that inspector error, either the Type 1 error of rejecting a good item or Type 2 error of accepting a faulty item, can lead to significant increase in the total cost of operating an SQC scheme. Models of human decision making were seen as a method by which the Human Factors specialist could predict the future performance of a quality control scheme by considering the possible changes in h u m a n behaviour with such factors as prior information, feed forward and feed back of information, or payoffs from the various outcomes. To this end, three theories were reviewed, namely Bayesian decision making, theory of selective information, and signal detection theory. The first two of these, while elegant theories and obviously models of how people make decisions, had very little predictive value in h u m a n inspection tasks. However, signal detection theory has been applied in a large number of inspection studies, inehiding a number of those presented at the symposium with encouraging results. It was pointed out, however, that the basic tenets of signal detection theory may not be applicable in any particular industrial inspection situation even though the data are available to calculate the parameters. It was suggested that until further evidence is forthcoming, this should be regarded as a descriptive model to predict the direction of performance rather than as a quantitative or normative model of how people do or should behave.
Signal detection S. Keith Adams, Department of Industrial Engineering, Oklahoma 244
Personnel selection In his review of individual factors and group differences in inspection, Earl L. Wiener of the University of Miami concluded that personnel selection, long a favourite of industrial management, offers little promise at present for improving the inspection task. The vast majority of studies both in laboratory and the field have shown little or no correlation between inspection performance and such variables as personality measures, visual acuity measures or age and sex of inspectors. Obvious correlations existed in some areas such as between visual search performance and peripheral visual acuity, or between vigilance performance and introversion/ extroversion score, but the main area for xmprovement appeared to be the area of employee motivation. I t is suggested that experiments with job enlargement and j o b enrichment would would probably pay higher rewards than improved selection testing.
Training David E. Embrey, University of Aston in Birmingham, UK, discussed both laboratory application of individual differences and the application of this to work in the inspection of photographic film for blemishes. His work with the photographic film inspection investigated two different methods of training a group of new operators: feedback training where inspectors were told whether alternate responses were correct detections, misses or false alarms; and cueing traimng where the subjects were told prior to each presentation of an item whether it was defective or good. It was found that, comparing these two groups with a
AppliedErgonomics December 1974
control group who merely practiced the task, there were n o statistically significant differences on correct detections, false alarms or signal detection theory measures. However. the response time did vary, with the cueing training being shown to be significantly superior to either the feedback training group or the control group in terms of speed of response. In the situation studied, speed of response was a particularly important variable as the stimulus only appeared on the screen for a very short period.
Lighting Improving performance by changing physical factors was reviewed by Terry W. Faulkner, Eastman Kodak Co, USA, who showed a n u m b e r of practical applications of changing the lighting for inspection tasks in order to improve performance. The major point made was that the t o t a l a m o u n t of light, usualiy specified by lighting engineers, was an inadequate measure of how good the lighting would be for inspection: it is much more the quality of the lighting than the quantity which affects performance. He reviewed a number of methods which have been used successfully, such as polarized lights, shadow graphing, spot lighting, diffuse and edge lighting. In all, 17 different types of lighting systems were described to provide a wide choice for the designer of inspection tasks. A case study on this was provided by George L. Smith, Ohio State University, who simulated the inspection of electronic chips. Such micro-miniature inspection was done using a microscope, and one of the major questions was what is the correct power of the microscope for best results. Test objects were produced by photographing patterns of rings and circles on 16mm film, so that the pattern to be scanned was very small and had to be scanned under a microscope. Using a stereo zoom microscope at different magnification, it was found that an 8x magnification was preferable in this study. Again, the analysis of the study in terms of signal detection theory produced interesting and useful results.
Work organisation The final method of improving performance was by the organization of the work. J.R. Buck of Purdue University showed the complexities inherent in organizing work based on a conveyor. It was found that conveyor velocity was not a complete description of performance, b u t that other factors such as viewing time, angle of view and viewing strategy need to be considered before performance can be
described. The visual inspection task was looking for faults in small discrete items passed along the conveyor belt, and was described as a sequence of four sub-tasks starting with observing the presence of an item in the viewing area, visual acquisition of the item, visual search within the item, and finally, decision making about the item. Again, it was suggested that signal detection theory was a useful model of the results.
Information The organization of information reaching the inspector was considered by X. K. Zunzanyika, Carborundum Co, Niagara Falls. Previous work had suggested that giving people either feedback information or feed forward information on faults should improve detection performance, but in a study at Carborundum, where a group receiving either feed forward, feedback or both types of information was compared with a control group, no significant differences were found. It was shown that both experimental and control groups increased in performance throughout the study, suggesting that merely running the study had improved performance even with experienced inspectors. It was also shown that the subjects adjusted their criterion as signal detection theory would suggest in response to changes in input quality. In summary, the symposium was seen as successful and thought provoking by both delegates and speakers. While the need is obvious for more work both in laboratory and field on inspection models, implementation of knowledge now existing would go a long way to improving inspection performance throughout industry. The conference proceedings are to be edited and published under the title
Human R eliability in Quality Control by Taylor and Francis Ltd. during the A u t u m n of 1974.
C.G. Drury
The factory and its people The Institution of Production Engineers, in assocation with the Ergonomics Research Society. The Royal Society, London. 17 September, 1974. The general theme of this conference was concisely expressed in its sub-title 'How production engineers have utilised ergonomics for the improvement of production activities'. An introduction to the techniques used was given by Prof E.N. Corlett,
Professor of Industrial Ergonomics in the Department of Engineering Production, University of Birmingham. Pointing out that it was necessary to use with the utmost effectiveness resources as expensive as people, he quoted examples showing the various areas in which ergonomics work can help: the lay-out of a workplace, the effect of the working environment, and psychological factors. Some gains can be quantified. Improvements to the design of a press guard increased output by some 12%, while redesigning a machine tool data panel reduced the number of errors, increased the learning rate and gave a 20% improvement in setting times. Away from the direct man-machine interface, it is less easy to demonstrate performance improvement, but Prof Corlett referred to the evidence available on the harmful effects of noise and heat and of ways to mitigate them. In one experimental example, an equivalent 30% extra fatigue suffered as a result of high temperature conditions was not compensated by 30% extra rest but by about 250%. Apart from the economic advantages of applying ergonomic techniques to increased productivity, their parallel application in improving working standards is being stimulated by social pressures, particularly in Sweden, and by legal forces. Obligatory standards in Germany require machines to be designed to the best ergonomic standards, and specifications are being laid down by 11 committees. There is also stronger trade union activity. Some 350000 copies of a 32 page book on the ergonomics of the workplace have been distributed to shop stewards in Germany, Austria and German speaking Switzerland. An Austrian union runs weekend courses, to which firms here asked to send staff representatives. In the USA, the recent Occupational Safety and Health Act puts considerable responsibility on management for the safety and security of workers and customers. Clearly, ignoring the indications and failing to take advantage of the techniques available could prove costly in terms of production costs, employment difficulties, and even lost orders through failure to offer acdeptable products.
Workplace problems The physical arrangement and size of a workplace is fundamental to any activity, and Mr R.S. Easterby, serdor lecturer and senior tutor in ergonomics in the Department of Applied Psychology at the University of Aston
in Birmingham, pointed out how the ergonomist's knowledge of anthropometry could be applied. A study carried out by Philips at Eindhoven showed that for musculoskeletal diseases, absenteeism amounted to 219 days per year per 100 men, and 177 days per year per 100 women. The study was made to establish a connection between symptoms of diseases of the musculoskeletal system and certain working postures. Of the 39 cases examined, the connection was considered 'certain' in 21 instances and 'probable' in 18 cases. Bad design was considered a factor in 80% of the cases and incorrect operator behaviour in 60% of the cases; these factors being combined in some instances. Mr Easterby outlined a n u m b e r of basic ergonomic elements in a workplace design, including the requirement for some movement but not long durations or frequent use of extreme positions, the avoidance of muscle overload and twisted or contorted postures, alternation between sitting and standing, with maximum times of 1 hour for sitting and half an hour for standing, the use of adjustable chairs, and training in working postures. He also illustrated the types of dimensional information available which were expressed in terms of median and standard deviation, and pointed out that it was usual to design for the 5th to 95th percentile. There were, of course, exceptions such as the need to design an aircraft escape hatch for the 100th percentile of the population involved, and for the positioning of a kitchen shelf at a height convenient for use by the shortest female. By using the data available, an envelope of acceptable distances for controls etc can be produced. Illustrating this point, Mr Easterby showed that in considering how near the operator should stand to a piece of equipment, there was a significant effect by the provision of a toe space some 4in (100 mm) deep by 3in (75 mm) high.
Effects of environment Prof Corlett concentrated on three aspects to illustrate work in this area. The first was the effects of high temperatures which could increase the heart rate by some 30 beats per minute over the normal near 70. High temperatures were often concealed by other poor working conditions. Besides producing fumes, welding could also be a source of radiant heat, while in a laundry the very high humidity was also accompanied by high temperatures and a necessity to work 'above heart level' through hanging clothes on frames. A graphic example was also
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Applications in industry Dr Alan D. S w a n of Sandia Laboratories, Alburquerque, New Mexico, gave the first paper on industrial applications. He showed how Human Factors engineers needed to be deeply involved in quality control aspects during the development of new high technology products. He discussed a number of studies, from his own experience, dealing with respecter accuracy and showing that this was never as high as was expected by management. One of the most successful methods which have been tried in his studies of assembling and testing complex electronic products was to use the 'do-check-do' method, where two operators alternate between assembling part of an item and inspecting the other's work. While this required more highly trained operators, it produced considerable improvement in product quality.
Steel industry The inspection problems m the steel industry were covered by Dr Jan Moraal of the Institute for Perception, Soesterberg, Netherlands. He analyzed the inspection of steel
242
sheets on a conveyor line by a number of different methods, ranging from an analysis of inspection reports filled in by the inspectors, through analysis of eye movements during on-line inspection, to analysis of errors in two laboratory simulations of the real task. All of these studies showed that a conveyor belt speed of 300 ft/min (91 m]min) was a critical point beyond which inspection performance suffered. Of considerable interest were his results on methods of training operators. He used two methods in a laboratory task, static training in which simulated sheets were presented statically to the inspectors, and dynamic training in which the sheets moved past the inspector with the same exposure time as in the static condition. Performance, measured by the signal detection theory index d', was found to be consistently better in the static condition than in the dynamic condition, with more improvement and higher overall performance. However, a second experiment in which half the subjects were deprived of a night's sleep before the testing showed that the static condition was much more drastically affected by sleep loss than the dynamic condition. The on-line inspection situation was improved by changing the workplace design, improving the lighting and reducing to four the number of fault categories into which inspectors had to sort.
Glass mspection G.J. Gillies of Pilkington Bros Ltd., St. Helens, UK, covered applications of Human Factors to inspection in the glass industry. His first application was inspection of moulded glassware for a long list of critical defects. The approach used here was to determine the optimum order of searching for faults, design a new lighting booth with specialized lighting for picking up each of the fault categories, and to institute a training method which increased in difficulty progressively with time both in the off-line situation and then in the on-line situation. A most important part of the implementation of the study was to involve both management and inspection personnel at all stages. In an extensive production trial with the new method, 15 inspectors missed less than 1% of the rejects which came to them. His second study was m inspection of sheet glass for bad cutting, consisting of either a corner missing
AppliedErgonomics December 1974
on a rectangular sheet of glass or a wrongly shaped corner put o n t o the glass. This was investigated using a simulation of glass cutting faults marked o n a roll of clear cellulose acetate pulled across an overhead projector and the image focused onto a white screen on the floor. A range of conveyor speed was studied, showing that a significant fall in performance occurred at a conveyor speed of about 3000 m/h. This was equivalent to a plate transit time of just under 2:s.
R u b b e r components Inspection o f small rubber components by R.W. Astley of the Lucas Institute for Engineering Production, Birmingham, UK: showed that the inspectors were rejecting a high proportion of good items (average of 9.6%). An improved inspection scheme was introduced in which the main emphasis was on redesign of the equipment and the workplace. Simple methods study techniques allowed almost a doubling of performance speed with no change in quality of inspection. The optical environment of the inspection was improved by using a greater power magnifying glass, and small fluorescent tubes to produce a reasonable light intensity without excess heat. Coupled with this was a change from a white laminated bench to a green surface to decrease the high contrast of the black rubber components. Because of the stimulating social environment in which the inspectors worked, it was not surprising that the introduction of background music, which has been found to work successfully elsewhere, produced no great improvement in performance. Food processing A. Sinclair, Loughborough University of Technology, UK, described two applications of Human Factors techniques to the food processing industry. The first was examination of trays of moulded pastry for jam tarts where on-line observation had indicated that insufficient time was being allowed to the inspectors to achieve proper inspection performance. An off-line simulation using real trays of jam tarts investigated the difference between stationary and moving presentation of the trays at three different exposure times per tray, 22-5. 15 s and 7-5 s. An analysis of the results of the experiment in terms both of hit and false alarm rates and of single
detection theory parameters showed that performance improved with increased presentation time. However, with the increased presentation time there was more opportunity for inspectors to throw away good items and this was reflected in a slightly higher false alarm rate. At the speeds used in this experiment there was no difference between moving and stationary presentations, in accordance with theoretical work which showed that there is no difference in moving and static presentation up to an angular velocity of 8°/s at the eye. The recommendations of the study included doubling the number of inspectors, a proposal which increased the detection rate of faulty items by approximately 5% and more than paid for the additional inspection s t a l l The second application described by Sinclair was the inspection of chicken carcasses in a chicken-packing factory. The job entailed taking from the the moving conveyor belt any carcass which was substandard because of bruising, broken limbs, etc. It was found that inspection standards were not rigidly defined but were determined to some extent by the state of the market. An experiment was designed with the conveyor moving at different speeds past inspectors on line at the same time of day, and this showed again that the percentage of rejects correctly identified improved with increasing presentation time up to approximately 1 s per bird. However, an anomalous finding was noted that above about 1¼ s per bird the hit rate decreased again. It was also noted in this experiment that performance steadily improved over the first 25 min of the task and then began to deteriorate in the manner expected with classical vigilance studies. To achieve good performance on the inspection task it was recommended that inspectors should be changed every 40 min during the day, and that the conveyor speed should be reduced wherever possible to allow at least 1 s per bird inspected.
Human performance models In the second day of the conference, three models of human performance apphcable to inspection were discussed, each model being presented and followed by a case study. Vigilance Classically, inspection has been regarded as a vigilance task so the first paper here was a presentation by Angus Craig, Medical Research Council Applied Psychology Unit, University of Sussex, UK, reviewing human performance models in vigilance, it was concluded that no unique theory had
emerged which would explain the experimental results found in studies of vigilance. The classical drop in performance with increased time on the job (the vigilance decrement) could either be explained as the inspectors' reduced expectancy for finding defects or signals or, on the other hand, by his reduced level of arousal. Despite this, many practical factors affecting vigilance performance have been found in laboratory studies. These include the effect of environmental conditions, such as ambient temperature or noise level, or of the time of day at which the inspection is performed, there being a larger drop in performance when inspectors are working during the night. Personality variables such as introversion/extraversion also appear to affect the results, with extraverts appearing to benefit from increased social interaction during the task. The major task related variables include the a priori probability that an item passing the inspector is defective: if an increased probability of defective occurs then performance improves. This has led to a number of recommendations that 'artificial signals' could be introduced into the inspection task to improve the arousal level of the inspector and, hence, his inspection performance. "t was pointed out by Dr Craig, however, that ff artificial signals were used, then full feedback of results had to be given Partial reinforcement by feedback produced a drop in performance when the feedback was removed.
Background music Recent studies have shown that a considerable proportion of the vigilance decrement observed in practice is due more to a shift in the subject's criterion than a change in his ability to discriminate good from bad product per se. As a case study in vigilance, J.G. Fox of the Ergonomics Information Analysis Centre, University of Birmingham, UK, discussed a study in a manufacturing company making small metal fasteners. Batches of metal fasteners were visually inspected to remove three types of defect. They were inspected under three conditions; first, normal working conditions with an ambient background noise of 83 dB; secondly, normal working conditions with a background of vocal music played for 10 min every 30 min; and thirdly, in a quiet condition where the background noise was reduced to 60dB(A). The results showed that there was a 7" 1% increase in fault detection under conditions of music. This was interpreted as being due to the increased arousal level of the inspectors
in the music condition over and above the other two conditions. In a second study in another factory, when silent conditions were compared with either unselected music or lively music, there was a progressive increase in fault detection with the music condition and accompanying this a reduction in the performance decrement in the second half-hour of work. Again, this confirms that arousal theory may be one of the keys to maintaining inspection performance. In the discussion of this study it was noted that if the music was changed to reflect an individual's personal preferences the fault detection scores were higher. However, it was suggested that if a resident disc jockey were employed to answer requests for music this might well be more distracting and meaningful than the task of inspection and lead to a decrement in performance. Visual search The theory of visual search, as presented by John R. Bloomfield of Honeywell Inc, USA, describes the situation in which an inspector must search either a regular or an irregular array of products, or even large sheet products, in order to locate and decide about individual defects. Bloomfield's review covered models of searching strategy ranging from completely systematic scanning, through systematic scanning with imperfect fault detection, to completely random scanning. It appears that most subjects" have a particular search strategy but may not follow this very exactly from trial to trial. All the models predict that as more time is allowed for the search there will be a higher probability of finding a defect, but the curves relating probability to time found in practice are reasonably insensitive to the model used. In an attempt to produce a predictive model as well as a descriptive model, Bloomfield's own work on search was discussed. This model describes accurately the search times obtained in looking for deviant items among an array of similar items or looking for small items on blank fields. It was deduced from the theoretical work that for good inspection one should choose inspectors with good peripheral acuity and that it may be possible to train inspectors to scan display material more efficiently. It was also shown that the search strategy will depend on the particular fault being looked for, so that if a number of faults need to be detected in the same area there is probably no single most efficient search strategy for them all.
The case study in visual search was also provided by Bloomfield. He took
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this opportunity to attempt a classification of the literature in visual search and inspection to produce the first step in a taxonomy of inspection tasks. This was considered by most people in the conference to be a most essential step in understanding and predicting performance in inspection. The fact that particular theories only apply to small parts of the task and that conflicting findings occur on different inspection tasks is probably due to the fact that the different inspection tasks emphasize different sorts of behaviour on the part of the inspector. It was emphasized that breaking down an inspection task using such a t a x o n o m y would have great advantages in field work on inspection.
State University, continued the exposition of signal detection theory in his case study paper. He showed how this theoretical framework could be used to study the inspection of small pieces of wire for cuts, the inspection of micro-miniature Landhott rings on a binocular microscope, and to elucidate visual search for letters. He introduced the concept that reaching a correct decision of accepting or rejecting an item involved first reaching a correct decision on whether or not to observe the item at all. There is no reason why both this decision and the decision on quality once observation is made cannot both be treated by signal detection theory.
Decision making
I nsl~etion performanceimprovement On the final day, a number of different approaches to improving inspection performance were covered.
Human decision making in quality control was reviewed by C.G. Drury of the State University of New York at Buffalo. Recent research in sampling quality control (SQC) plans had shown that inspector error, either the Type 1 error of rejecting a good item or Type 2 error of accepting a faulty item, can lead to significant increase in the total cost of operating an SQC scheme. Models of human decision making were seen as a method by which the Human Factors specialist could predict the future performance of a quality control scheme by considering the possible changes in h u m a n behaviour with such factors as prior information, feed forward and feed back of information, or payoffs from the various outcomes. To this end, three theories were reviewed, namely Bayesian decision making, theory of selective information, and signal detection theory. The first two of these, while elegant theories and obviously models of how people make decisions, had very little predictive value in h u m a n inspection tasks. However, signal detection theory has been applied in a large number of inspection studies, inehiding a number of those presented at the symposium with encouraging results. It was pointed out, however, that the basic tenets of signal detection theory may not be applicable in any particular industrial inspection situation even though the data are available to calculate the parameters. It was suggested that until further evidence is forthcoming, this should be regarded as a descriptive model to predict the direction of performance rather than as a quantitative or normative model of how people do or should behave.
Signal detection S. Keith Adams, Department of Industrial Engineering, Oklahoma 244
Personnel selection In his review of individual factors and group differences in inspection, Earl L. Wiener of the University of Miami concluded that personnel selection, long a favourite of industrial management, offers little promise at present for improving the inspection task. The vast majority of studies both in laboratory and the field have shown little or no correlation between inspection performance and such variables as personality measures, visual acuity measures or age and sex of inspectors. Obvious correlations existed in some areas such as between visual search performance and peripheral visual acuity, or between vigilance performance and introversion/ extroversion score, but the main area for xmprovement appeared to be the area of employee motivation. I t is suggested that experiments with job enlargement and j o b enrichment would would probably pay higher rewards than improved selection testing.
Training David E. Embrey, University of Aston in Birmingham, UK, discussed both laboratory application of individual differences and the application of this to work in the inspection of photographic film for blemishes. His work with the photographic film inspection investigated two different methods of training a group of new operators: feedback training where inspectors were told whether alternate responses were correct detections, misses or false alarms; and cueing traimng where the subjects were told prior to each presentation of an item whether it was defective or good. It was found that, comparing these two groups with a
AppliedErgonomics December 1974
control group who merely practiced the task, there were n o statistically significant differences on correct detections, false alarms or signal detection theory measures. However. the response time did vary, with the cueing training being shown to be significantly superior to either the feedback training group or the control group in terms of speed of response. In the situation studied, speed of response was a particularly important variable as the stimulus only appeared on the screen for a very short period.
Lighting Improving performance by changing physical factors was reviewed by Terry W. Faulkner, Eastman Kodak Co, USA, who showed a n u m b e r of practical applications of changing the lighting for inspection tasks in order to improve performance. The major point made was that the t o t a l a m o u n t of light, usualiy specified by lighting engineers, was an inadequate measure of how good the lighting would be for inspection: it is much more the quality of the lighting than the quantity which affects performance. He reviewed a number of methods which have been used successfully, such as polarized lights, shadow graphing, spot lighting, diffuse and edge lighting. In all, 17 different types of lighting systems were described to provide a wide choice for the designer of inspection tasks. A case study on this was provided by George L. Smith, Ohio State University, who simulated the inspection of electronic chips. Such micro-miniature inspection was done using a microscope, and one of the major questions was what is the correct power of the microscope for best results. Test objects were produced by photographing patterns of rings and circles on 16mm film, so that the pattern to be scanned was very small and had to be scanned under a microscope. Using a stereo zoom microscope at different magnification, it was found that an 8x magnification was preferable in this study. Again, the analysis of the study in terms of signal detection theory produced interesting and useful results.
Work organisation The final method of improving performance was by the organization of the work. J.R. Buck of Purdue University showed the complexities inherent in organizing work based on a conveyor. It was found that conveyor velocity was not a complete description of performance, b u t that other factors such as viewing time, angle of view and viewing strategy need to be considered before performance can be
described. The visual inspection task was looking for faults in small discrete items passed along the conveyor belt, and was described as a sequence of four sub-tasks starting with observing the presence of an item in the viewing area, visual acquisition of the item, visual search within the item, and finally, decision making about the item. Again, it was suggested that signal detection theory was a useful model of the results.
Information The organization of information reaching the inspector was considered by X. K. Zunzanyika, Carborundum Co, Niagara Falls. Previous work had suggested that giving people either feedback information or feed forward information on faults should improve detection performance, but in a study at Carborundum, where a group receiving either feed forward, feedback or both types of information was compared with a control group, no significant differences were found. It was shown that both experimental and control groups increased in performance throughout the study, suggesting that merely running the study had improved performance even with experienced inspectors. It was also shown that the subjects adjusted their criterion as signal detection theory would suggest in response to changes in input quality. In summary, the symposium was seen as successful and thought provoking by both delegates and speakers. While the need is obvious for more work both in laboratory and field on inspection models, implementation of knowledge now existing would go a long way to improving inspection performance throughout industry. The conference proceedings are to be edited and published under the title
Human R eliability in Quality Control by Taylor and Francis Ltd. during the A u t u m n of 1974.
C.G. Drury
The factory and its people The Institution of Production Engineers, in assocation with the Ergonomics Research Society. The Royal Society, London. 17 September, 1974. The general theme of this conference was concisely expressed in its sub-title 'How production engineers have utilised ergonomics for the improvement of production activities'. An introduction to the techniques used was given by Prof E.N. Corlett,
Professor of Industrial Ergonomics in the Department of Engineering Production, University of Birmingham. Pointing out that it was necessary to use with the utmost effectiveness resources as expensive as people, he quoted examples showing the various areas in which ergonomics work can help: the lay-out of a workplace, the effect of the working environment, and psychological factors. Some gains can be quantified. Improvements to the design of a press guard increased output by some 12%, while redesigning a machine tool data panel reduced the number of errors, increased the learning rate and gave a 20% improvement in setting times. Away from the direct man-machine interface, it is less easy to demonstrate performance improvement, but Prof Corlett referred to the evidence available on the harmful effects of noise and heat and of ways to mitigate them. In one experimental example, an equivalent 30% extra fatigue suffered as a result of high temperature conditions was not compensated by 30% extra rest but by about 250%. Apart from the economic advantages of applying ergonomic techniques to increased productivity, their parallel application in improving working standards is being stimulated by social pressures, particularly in Sweden, and by legal forces. Obligatory standards in Germany require machines to be designed to the best ergonomic standards, and specifications are being laid down by 11 committees. There is also stronger trade union activity. Some 350000 copies of a 32 page book on the ergonomics of the workplace have been distributed to shop stewards in Germany, Austria and German speaking Switzerland. An Austrian union runs weekend courses, to which firms here asked to send staff representatives. In the USA, the recent Occupational Safety and Health Act puts considerable responsibility on management for the safety and security of workers and customers. Clearly, ignoring the indications and failing to take advantage of the techniques available could prove costly in terms of production costs, employment difficulties, and even lost orders through failure to offer acdeptable products.
Workplace problems The physical arrangement and size of a workplace is fundamental to any activity, and Mr R.S. Easterby, serdor lecturer and senior tutor in ergonomics in the Department of Applied Psychology at the University of Aston
in Birmingham, pointed out how the ergonomist's knowledge of anthropometry could be applied. A study carried out by Philips at Eindhoven showed that for musculoskeletal diseases, absenteeism amounted to 219 days per year per 100 men, and 177 days per year per 100 women. The study was made to establish a connection between symptoms of diseases of the musculoskeletal system and certain working postures. Of the 39 cases examined, the connection was considered 'certain' in 21 instances and 'probable' in 18 cases. Bad design was considered a factor in 80% of the cases and incorrect operator behaviour in 60% of the cases; these factors being combined in some instances. Mr Easterby outlined a n u m b e r of basic ergonomic elements in a workplace design, including the requirement for some movement but not long durations or frequent use of extreme positions, the avoidance of muscle overload and twisted or contorted postures, alternation between sitting and standing, with maximum times of 1 hour for sitting and half an hour for standing, the use of adjustable chairs, and training in working postures. He also illustrated the types of dimensional information available which were expressed in terms of median and standard deviation, and pointed out that it was usual to design for the 5th to 95th percentile. There were, of course, exceptions such as the need to design an aircraft escape hatch for the 100th percentile of the population involved, and for the positioning of a kitchen shelf at a height convenient for use by the shortest female. By using the data available, an envelope of acceptable distances for controls etc can be produced. Illustrating this point, Mr Easterby showed that in considering how near the operator should stand to a piece of equipment, there was a significant effect by the provision of a toe space some 4in (100 mm) deep by 3in (75 mm) high.
Effects of environment Prof Corlett concentrated on three aspects to illustrate work in this area. The first was the effects of high temperatures which could increase the heart rate by some 30 beats per minute over the normal near 70. High temperatures were often concealed by other poor working conditions. Besides producing fumes, welding could also be a source of radiant heat, while in a laundry the very high humidity was also accompanied by high temperatures and a necessity to work 'above heart level' through hanging clothes on frames. A graphic example was also
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