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Approaches to training for industrial inspection
Applied Ergonomics 1979, 10.3, 139-144
Ergonomics of visual inspection
Approaches to training for industrial inspection D.E. Embrey Applied Psychology Department, University of Aston in Birmingham
This paper reviews the techniques available for detection and recognition training, and their application to industrial inspection. Systematic procedures are described for using these techniques as part of an integrated training scheme. Approaches to task analysis for inspector training are also discussed.
Introduction
A systems approach to inspection training
Training for inspection is very much a neglected area both in industry and, more surprisingly, in inspection research. This neglect is Unfortunate, for several reasons. Improved training techniques can provide a very costeffective means of improving a quality control system. Training innovations do not require any extensive changes to the hardware of a system, and it is usually relatively easy to demonstrate the improvements in performance which can result from improved training methods. The magnitude of these improvements is often very much greater than can be achieved by other applications of ergonomics to inspection.
Ekstrand (1964) sets out a comprehensive general approach to the development of a training system. The following sections will illustrate the application of this approach to inspection training.
The general lack of interest in industry in inspection training (with some notable exceptions) is perhaps not surprising. Both training and inspection are frequently regarded as basically non-productive activities, which have to be tolerated as necessary evils. Very often it is assumed that a worker who has had experience in production will be able to carry out inspection without any specific training. In addition, the influence of work study approache: which tend to concentrate on externally observable behaviour, in largely manual tasks, has tended to undervalue the primarily perceptual skills of inspection. The neglect of the training area by inspection researchers is less easy to understand. The training literature in general has been dominated by work on tasks which have a high motor content. This probably reflects the perceived training needs of industry in the past. However, increasing automation is emphasising the importance of training for jobs which involve cognitive, decision making and recognition skills. Many forms of inspection involve these skills. Before considering specific training techniques a general approach to inspection training will be discussed.
This article continues a short series on the general topic of Ergonomics of visual inspection, begun on page 16 of the March issue,under the general editorship of E.D. Megaw.
0003-6870/79/03 139 06 $02.00 O IPC Business Press
1. Definition of training objectives In the case of inspection, the definition of the goals of training would appear to be quite straightforward. Most Quality Control managers would probably define an ideal inspector as one who detects every defect. This might therefore be regarded as the objective of any training system. In fact, this is something of an over-simplification. The Signal Detection Theory approach to inspection discussed in Sinclair (1979) and Drury and Fox (1975) suggests that the detection of every defect is an unrealistic goal, and that increases in detection probability may also increase the likelihood of rejecting good product ('false alarms'). The training objectives therefore need to take into account these and other factors. For example, in some situations it may not be important if the inspector rejects a small proportion of good items, as long as very few defects are missed. In other cases, speed of inspection may be important. It is also important at this stage to define as far as possible both the characteristics of acceptable product and the range of defects likely to be encountered. If the quality control system as a whole lacks clear definitions with regard to the attributes of acceptable quality, then clearly the training of inspectors is unlikely to be very effective.
2. Development of criterion measures The criteria to which inspectors are trained will obviously reflect the overall objectives of the quality control system as defined in the first stage. The question of performance criteria can be complex. In an earlier paper in this series, Sinclair (1979) discussed a number of performance measures which have been used in inspection research. No single index will be applicable to all situations, but it is likely that both the probability of correct defect detection and also
Applied Ergonomics September 1979
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the probability of rejecting good product need to be specified as part of the training criteria. Other factors could also be included, and a complete set of criteria might produce the following specification (this example is hypothetical) : The inspector shall achieve the following performance standards: (a)
95% of all defects to be detected in the standard test batch (1000 items, containing 50 defects) with rejection of good product to be less than 1%.
(b)
The standards in (a) to be achieved at an average inspection rate of 250 items/h.
(c)
Standards (a) and (b) shall be maintained for at least 2 h of continuous inspection.
(d)
Increases in the incidence of defects of more than 25% during any half hour period should be detected.
(e)
The defined procedures to be followed on the occurrence of defect categories x, y and z should be correctly carried out.
This example illustrates the range of performance characteristics which might be included in the training criteria. A detailed specification of this kind provides a clear overview of the training requirements for the system.
complementary to Hierarchical Task Analysis, is to use a simple sequential model of the inspection task such as that described in Embrey (1976). This model considers the successive operations required to carry out inspection. The first stage is sensing, where the inspector examines the product to acquire the sensory evidence necessary to decide whether it is defective. The next stage is detecti0n/recognition, a decision making process where the evidence derived from the first stage is compared with some form of mental specification of an acceptable item. At this stage the item may be categorised into a class such as acceptable, reject, reject but mendable, etc. Finally, there is an action phase where the appropriate response is selected, eg, place the item on the 'acceptable' conveyor, send for rectification, etc. The advantage of this approach is that it serves to direct the training analyst to the area ,of research which is appropriate for a particular training problem, This model could be used in conjunction with the Hierarchical Task Analysis described earlier. Whichever technique is applied, the results of the task analysis will enable the content of the training schenie to be specified, as well as providing the training analyst with a detailed appreciation of the task.
Constituents of an inspection training programme 3. Derivation of training content This stage involves the specification of the constituents of the training programme required to meet the criteria defined
Three basic requirements can be identified as being of particular importance for effective inspection.
in (2).
(a) (b)
The attitudes of the inspector to the work. Job knowledge.
4. Designof training methods and materials
(c)
Possession of the specific skills required to perform the task.
The constituents of the programme set out at stage (3) will be translated into practical training materials at this stage. The specific techniques available will be considered in detail in later sections of this paper. 5. The training programme At this stage, the potential inspectors will encounter the training programme, Different forms of training programme may be necessary for workers of different backgrounds, eg, new employees, or existing workers being retrained. The performance of graduates from the training programme is compared with the system criteria. During this validation phase, modifications may be made to the programme in order to achieve the objectives of the system.
Task analysis procedures Task analysis is a systematic procedure which is used for defining training objectives and specifying training content. Task analysis is partly descriptive, in that it delineates the nature of the task in an orderly way, at varying levels of detail. It attempts to isolate the critical characteristics of the task for which specific training techniques will be appropriate. The prescriptive aspect of task analysis provides the basic framework for the training content. One particular form of task analysis which can be applied to inspection tasks is the Hierarchical Task Analysis of Annett and Duncan (1967). This approach successively redescribes a task at increasing levels of detail. At each level, particular aspects of the task likely to require specific training methods are isolated. Another approach, which may be regarded as being
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Attitudinal factors are of particular importance in inspection. Because the inspector is usually the last person to see the product before it reaches the customer, his attitude to his work wilt directly influence the quality image of the company. Unfortunately, in this country, inspection is often regarded as an unproductive, and therefore low status, job. Training, and indeed company policy as a whole, should be directed towards emphasising the importance of the inspector, whose status and remuneration should reflect this importance. There are several types of job knowledge require d for inspection work. The worker has to learn the basic protocols and procedures required to carry out the task. eg, where to find the items to be inspected, what to do when particular types of defect are encountered, and the paper work required. A different kind of job knowledge is required to recognise the different categories of defect which will occur. An appreciation of the manufacturing processes involved in producing the product and their effects on the types of defect likely to occur, is also important. There is a distinction to be made between the knowledge required for recognition and other types of job knowledge. Factual knowledge can usually be explicitly defined. On the other hand, it is often only possible to give examples of specific defects. The inspector has to learn to generalise from these examples to the types of defect actually encountered in the real inspection task. The final requirement is the possession of the specific skills required to perform the task. In the case of inspection, these are primarily the decision making and perceptual skills involved in the detection and recognition of defects.
Although a training programme will include each of the three areas discussed up to now, the remainder of this review will concentrate on the last area, the development of the specific skills required for inspection. This emphasis is a reflection of the relative amount of work which has been carried out in these areas. Very few studies exist in which attitudinal and motivational variables have been investigated. Some of these studies are discussed in Wiener (1975). On the other hand, there has been some investigation of training approaches to develop recognition skills, which will be discussed in detail in the next section. The overall emphasis of the remainder of this review will be towards training techniques applicable to inspection tasks requiring a high degree of subjective judgement of quality. The types of industrial inspection involving simple go/no-go decisions, using gauges or instruments, can usually be trained for using procedures similar to those for simple manual tasks.
Training approaches to inspection skills The following review will first consider the principles which have emerged regarding training for detection and recognition skills in general. Much of this work was carried out in the context of auditory detection in sonar applications. Nevertheless, the consensus of opinion is that these results can be readily generalised to inspection tasks. The second part of the review will describe some applied studies of inspection tasks or situations which closely resemble them. The large body of studies which analyse training for vigilance tasks will not be discussed, because it is considered that most real industrial inspection tasks are very dissimilar from the classical vigilance situation.
Knowledge of results and cuing One of the most general principles which has emerged from research in detection and recognition skills is the importance of feedback or knowledge of results (KR) in the acquisition of these skills. KR has been defined as "knowledge which an individual or group receives relating to the outcome of a response or group of responses", Annett (1961). In the context of inspection training, KR can be used in several ways. The usual method is to present the trainee with a batch of items, some of which contain characteristic defects. The trainee attempts to identify each item as defective or otherwise, and is given immediate feedback as to the correctness of his decision after each response. In some cases, he is told whether he made a correct decision or false alarm, or sometimes only information regarding correct detections is given. KR has been shown to be an essential requirement for almost all types of learning, Annett (1969). It is usually regarded as promoting learning in three ways: (a)
(b)
(c)
Its motivational effect. By providing the learner with information regarding his progress, KR encourages him to remain in the learning situation.
regarding the critical differences between defects and perfect items. Some workers have suggested that perceptual learning may occur via different mechanisms than those operating in motor learning, eg, Gibson (1953). In a series of studies, Annett and Clarkson (1964), Annett and Paterson (1966, 1967) proposed that perceptual learning took place via the simple pairing of a stimulus and its name. This implies that an overt response on the part of a trainee is not really necessary. Essentially, Annett suggested that the observer built up a mental 'template', which was an internal representation of the defining characteristics of defective items. This implied that the informational content of KR was of particular importance. Consistent with this approach, the technique of 'cuing' was advocated as an alternative to KR. Cuing involved the presentation of a series of examples of signals (defects) and non-signals (perfect items). Prior to each presentation, the trainee was told whether the next item was going to be a signal or otherwise, and no overt response was required. The warning was said to alert the learner, thus enabling him to attend more carefully to the defect characteristics. This approach seemed to be equal, or in some cases, superior in effectiveness, to conventional KR for many of the tasks considered by Annett. With later experiments, using more complex stimuli, Annett and Paterson (1967) found that cuing and KR were equivalent in their training effect, when they provided the same degree of information. This can be interpreted as providing support to the 'template' approach to perceptual learning. In practical terms, the choice between cuing and KR seems to be largely a matter of taste, although some workers have suggested that trainees can become 'cue dependent', ie, their performance deteriorates when the cue is removed. The gradual reduction of the level of cuing during training ('lading') has been used to reduce this effect.
Whole and feature-analysis approaches to perceptual training Two basic approaches to perceptual training can be identified. In the first of these, the 'whole' approach, the pattern to be recognised is used more or less intact from the beginning of training. This is the approach used in the studies described in the last section. In the alternative 'feature-analysis' approach, recognition is said to involve the synthesis of separate aspects or features of the pattern, or defect. Wallis (1963) gives a comprehensive description of an example of this approach. Wallis described his procedure as an analytic-synthetic methodology. Essentially, the salient characteristics of the defects are first learnt separately, and then brought together during training, until the signal can be recognised as a whole. The main features of Wallis's approach can be summarised as follows: 1. The emphasis is placed on realistic teaching being used. Even at the intial stages of training, the items used are real examples or exact facsimilies.
The reinforcement of observing responses. The observing response is an information seeking response on the part of the observer. This encourages him to actively examine the items being inspected.
2. Tile early stages of training attempt to distinguish between the characteristics of the non-defects and the cues characteristic of defects.
Its informational content. Many learning theorists regard this as the most important attribute of KR. KR provides information feedback to the trainee
3. Graded training exercises are given such that the cues involved in discrimination are initially very easy to discern, and are presented one at a time. Subsequently,
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the exercises are increased in difficulty until the cues are more representative of those encountered in the real task. KR is used at this stage to enahnce discrimination and to reduce false alarms. 4. An additional object of the training exercises is to accustom the learners to responding under paced conditions. Many recognition tasks allow only a limited sampling time in which to acquire the information needed to make a .discrimination (eg, paced conveyor belt inspection). 5. During the early training exercises the learner provides a verbal commentary regarding the cues being acquired, and their significance in the total recognition process. 6. Finally, the emphasis of the training changes from an analytic to a synthetic approach. Instead of responding to separate cues, the trainee has to make an overall judgement based on all the evidence available. WaUis emphasises the importance of KR in this approach, and suggests that feedback should be 'frequent, detailed and immediate'. He warns that although supplementary cues and information may be useful in the early stages of learning, they must be withdrawn later to prevent cue dependance. The validity of several of the techniques employed by Wallis is supported by other work. Gibson (1953) also found that experience with realistic material was important, and that learning could occur simply by exposure to the material. Mackie and Harabedian (1964) found transfer of training more effective when authentic material was used.
The relative effectiveness of different forms of training Gibson (1947) had found that whole methods were slightly less efficient than feature analysis approaches. However, studies by Allan (1957, 1958) comparing two training techniques for aircraft recognition, came to the opposite conclusion. Annett (1971) carried out an extensive series of experiments comparing the two approaches in a sonar recognition task. In all, 15 different training methods were compared. The overall conclusions were that there was no particular advantage in using feature analysis approaches compared with simple whole methods. Both techniques produced a substantial training effect. The most important single factor seemed to be the sheer experience of named samples of the various types of signal to be identified. The study also found that there was no particular advantage in ordering the material in terms of difficulty. With regard to the verbal explanations used by instructors, it was found that simple instructions were most effective. Also, it was best to concentrate on the one best cue which characterised the signal (defect). Detailed instructions about a variety of cues could not be dealt with effectively and could even depress performance.
Applied studies of training for inspection Although the studies considered in this section are not authentic inspection tasks, they share many of the characteristics of such tasks, eg, complex items. Learning effects in laboratory simulations of inspection tasks have taken place without any specific training being given, eg, Smith and Adams (1971) and Lion et al (1968). This tends to confirm Annett's assertion that learning can occur
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simply by exposure to the material to be learnt. In an industrial setting, Chancy and Tee1 (1967)have used several approaches in training machined parts and photomask inspectors. Their most commonly applied technique, known as job sample instruction, involves providing the inspectors with KR after inspecting test items containing typical defects. Martinek and Sadacca (1965) report a study in training photo-interpreters to identify targets on aerial photographs. Martinek provided the photo-interpreters with an error key which analysed the commonly occurring errors of previous interpretations. This produced fewer errors of commission than a key in which the the characteristic features of the various target types were set out. The error key can be regarded as a form of group KR. Another photo-interpretation study, Cockrell and Sadacca (1971) showed that KR was more effective in a team context when team members first scanned a photograph independently and then discussed the results together immediately afterwards. The greatest gains in proficiency were made by the least able members of the team. Although detection performance was improved significantly, there were greater gains in reducing the number of misidentifications and false alarms. Powers et al (1973) attempted to improve performance in this task by modifying the interpreters' scanning strategies. Structured search practice increased the number of target detections at the expense of a greater number of false alarms. A 'speed reading' training technique, designed to reduce fixation times and expand the visual field, halved the search time. but did not produce any inc tease in accuracy. Training using the error key approach discussed earlier significantly reduced false alarms. Brock. Wells, and Abrams (1974) provided a complete training programme for the examiners of radiographs used in non-destructive testing. The programme consisted of tape/slide presentations which gave examples of various types of defect, and then a KR phase where radiographs containing defects were presented and the student given full KR after he had attempted to identify them. A selfadministered test then determined if the student should go on to the next module or re-take prevtous modules. There was a gradual increase in difficulty of the radiographs both within and between modules. The programme was highly successful, in that it reduced the time needed to train to the required criterion from an average to 80 h to 10-9 h.
Research on inspection training: general conclusions It is difficult to evaluate the practical applicability of much of the research discussed in earlier sections because of the very limited number of studies which have used authentic industrial inspection tasks, Nevertheless, the applied studies which have been carried out have tended to confirm the laboratory based findings. Most of the studies agree on the importance of allowing the trainee to build up a 'mental model' of the characteristics of good and defective product. This seems to be achieved by extensive experience with authentic examples of defects and non-defects. Both cuing and KR seem equally effective in this. Similarly. both whole and part approaches appear to promote equal degrees of learning. Where the task being trained for is paced, this should be explicitly considered in the training programme.
The instructional style to be used during training is also important. Annett's findings that simple instructions, dealing with one cue at a time, were most effective seem relevant here. Virtually no formal work has been carried out on the long term retention of inspection skills, although Thomas {1962) discussed the importance of recalibrating the inspector to reduce the possibility of drifts in standards over time. The most effective form of this type of retraining needs to be established. Nearly all the training techniques discussed up to now have been designed to enhance the inspector's sensitivity(d') in Signal Detection Theory terms. Although sensitivity is probably the most important factor, there remains the possibility that the inspector might be trained to employ the most efficient rejection criterion. Signal Detection Theory suggests that this is defined by the probability of a detect occurring, and the relative costs and values of rejecting good product or missing defects. Embrey (1976) carried out some preliminary experiments which suggested that some forms of training do enable the inspector to adjust the criterion more effectively in accordance with changing defect percentages. This work requires elaboration before being included in inspection training.
Conclusions The consideration of a large body of research has produced some practical guidelines for training inspectors. These need to be implemented within a systematic procedure for the development of an inspection training scheme. The procedures of Eckstrand, set out in the early sections of this paper, can be used for this purpose. The importance of a systematic task analysis, to enable the techniques appropriate for a particular situation to be used, needs to be re-emphasised. Finally, it is hoped that in the future the ergonomist will be involved in the design of quantity control systems from the outset, in order to apply ergonomics principles in the most cost-effective way.
References Allan, M.D. 1957 OccupationalPsychology, 31, 113-119. Training in perceptual skills.
Allan, M.D. 1958 OccupationalPsychology, 32,245 252. Learning perceptual skills: the Sargeant system of recognition training.
Annett, J. 1961 The role of knowledge of results in learning: A survey. NAVTRADEVCEN Technical Report No 342 3, US Naval Training Device Center, New York.
Annett, J., and Clarkson, J.K. 1964 The use of cuing in training tasks. Technical Report 3143 -1, US Naval Training Device Center.
Annett, J., and Duncan, K.D. 1967 OccupationalPsychology, 4 I, 211 22 l. Task analysis and training design.
Annett, J., and Paterson, L. 1966 The use of cuing in training tasks: Phase II. Technical Report 4119 1, US Naval Training Device Center.
Annett, J., and Paterson, L. 1967 The use of cuing in training tasks: Phase III. Technical Report, NAVTRADEVCEN 4717 1, Orlando, Florida.
Brock, J.F., Wells, R.G., and Abrams, M.L. 1974 Development and validation of an experimental radiograph reading training program. NPRDC-TR-74-33, Navy Personnel Research and Development Center, San Diego.
Chancy, F.B., and Ted, T.S. 1967 Journal of Applied Psychology, 51, 3 l 1 315. Improving inspector performance through training and visual aids.
Cockrell, J.T., and Sadacca, R. 1971 Training individual image interpreters using team concensus feedback. US Army Behavior and Systems Research Laboratory, Technical Research Report 1171. Drury, C.G., and Fox, J.G. (eds) 1975 Human reliability in quality control. London: Taylor and Francis.
Eckstrand, G.A. 1964 Current status of the technology of training. Wright-Patterson AFB Aerospace Medical Laboratory, Report AMRL-TRR-64-86.
Embrey, D.E. 1976 Signal detection theory in the analysis and optimisation of industrial inspection tasks. Unpublished PhD thesis, University of Aston in Birmingham.
Gibson, J.J. 1947 The possibilities of research on training in recognition and other visual performances. Report to the Armed Forces by the NRC Vision Committee, University of Michigan.
Gibson, E.G. 1953 PsychologicalBulletin, 50, 401 - 431. Improvement in perceptual judgements as a function of controlled practice or training.
Lion, J.S., Richardson, E., and Browne, R.C. 1968 Ergonomics, 2, 23--34. A study of the performance of industrial inspectors under two kinds of lighting.
Annett, J.
Mackie, R.R., and Harabedian, A.
1969 Feedback and human behaviour. Harmondsworth: Penguin.
1964 A study of simulation requirements for sonar operator trainees, US Naval Training Device Center, Technical Report No 1320 1.
Annett, J. 1971 Sonar recognition training: An investigation of whole versus part and analytic versus synthetic procedures. Technical Report No 67-C-0105-1, US Naval Training Device Center, Orlando.
Martinek, H., and Sadacca, R. 1965 Error keys as reference aids in image interpretation. US Army Personnel Research Office, Washington. Technical Research Note 153.
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Powers, J.R., Brainard, R.W., Abram, R.E,, and Sadaeea, R. 1973 Training techniques for rapid target detection. US Army Research Institute for the Behavioral and Social Sciences, Technical Paper 242~
Sinclair, M.A. 1979 Applied Ergonomics, 10, 17-25. The use of performance measures on individual examiners in inspection schemes.
Smith, G.L., and Adams, S.K. 1971 Human Factors, 13, 247-254. Magnification and rnicrominiature inspection.
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Applied Ergonomics September 1979
Thomas, L.F. 1962 Ergonomics, 5, 429-434. Perceptual organisation in industrial inspectors.
Wallis D.
1963 OccupationalPsychology, 37, 237-254. Occupational psychology in the sixties: some implications of recent studies of perceptual training and skill.
Wiener, E.L. 1975 Individual and group differences in inspection. In: Human reliability and quality control. London: Taylor and Francis.
Ergonomics of visual inspection
Approaches to training for industrial inspection D.E. Embrey Applied Psychology Department, University of Aston in Birmingham
This paper reviews the techniques available for detection and recognition training, and their application to industrial inspection. Systematic procedures are described for using these techniques as part of an integrated training scheme. Approaches to task analysis for inspector training are also discussed.
Introduction
A systems approach to inspection training
Training for inspection is very much a neglected area both in industry and, more surprisingly, in inspection research. This neglect is Unfortunate, for several reasons. Improved training techniques can provide a very costeffective means of improving a quality control system. Training innovations do not require any extensive changes to the hardware of a system, and it is usually relatively easy to demonstrate the improvements in performance which can result from improved training methods. The magnitude of these improvements is often very much greater than can be achieved by other applications of ergonomics to inspection.
Ekstrand (1964) sets out a comprehensive general approach to the development of a training system. The following sections will illustrate the application of this approach to inspection training.
The general lack of interest in industry in inspection training (with some notable exceptions) is perhaps not surprising. Both training and inspection are frequently regarded as basically non-productive activities, which have to be tolerated as necessary evils. Very often it is assumed that a worker who has had experience in production will be able to carry out inspection without any specific training. In addition, the influence of work study approache: which tend to concentrate on externally observable behaviour, in largely manual tasks, has tended to undervalue the primarily perceptual skills of inspection. The neglect of the training area by inspection researchers is less easy to understand. The training literature in general has been dominated by work on tasks which have a high motor content. This probably reflects the perceived training needs of industry in the past. However, increasing automation is emphasising the importance of training for jobs which involve cognitive, decision making and recognition skills. Many forms of inspection involve these skills. Before considering specific training techniques a general approach to inspection training will be discussed.
This article continues a short series on the general topic of Ergonomics of visual inspection, begun on page 16 of the March issue,under the general editorship of E.D. Megaw.
0003-6870/79/03 139 06 $02.00 O IPC Business Press
1. Definition of training objectives In the case of inspection, the definition of the goals of training would appear to be quite straightforward. Most Quality Control managers would probably define an ideal inspector as one who detects every defect. This might therefore be regarded as the objective of any training system. In fact, this is something of an over-simplification. The Signal Detection Theory approach to inspection discussed in Sinclair (1979) and Drury and Fox (1975) suggests that the detection of every defect is an unrealistic goal, and that increases in detection probability may also increase the likelihood of rejecting good product ('false alarms'). The training objectives therefore need to take into account these and other factors. For example, in some situations it may not be important if the inspector rejects a small proportion of good items, as long as very few defects are missed. In other cases, speed of inspection may be important. It is also important at this stage to define as far as possible both the characteristics of acceptable product and the range of defects likely to be encountered. If the quality control system as a whole lacks clear definitions with regard to the attributes of acceptable quality, then clearly the training of inspectors is unlikely to be very effective.
2. Development of criterion measures The criteria to which inspectors are trained will obviously reflect the overall objectives of the quality control system as defined in the first stage. The question of performance criteria can be complex. In an earlier paper in this series, Sinclair (1979) discussed a number of performance measures which have been used in inspection research. No single index will be applicable to all situations, but it is likely that both the probability of correct defect detection and also
Applied Ergonomics September 1979
139
the probability of rejecting good product need to be specified as part of the training criteria. Other factors could also be included, and a complete set of criteria might produce the following specification (this example is hypothetical) : The inspector shall achieve the following performance standards: (a)
95% of all defects to be detected in the standard test batch (1000 items, containing 50 defects) with rejection of good product to be less than 1%.
(b)
The standards in (a) to be achieved at an average inspection rate of 250 items/h.
(c)
Standards (a) and (b) shall be maintained for at least 2 h of continuous inspection.
(d)
Increases in the incidence of defects of more than 25% during any half hour period should be detected.
(e)
The defined procedures to be followed on the occurrence of defect categories x, y and z should be correctly carried out.
This example illustrates the range of performance characteristics which might be included in the training criteria. A detailed specification of this kind provides a clear overview of the training requirements for the system.
complementary to Hierarchical Task Analysis, is to use a simple sequential model of the inspection task such as that described in Embrey (1976). This model considers the successive operations required to carry out inspection. The first stage is sensing, where the inspector examines the product to acquire the sensory evidence necessary to decide whether it is defective. The next stage is detecti0n/recognition, a decision making process where the evidence derived from the first stage is compared with some form of mental specification of an acceptable item. At this stage the item may be categorised into a class such as acceptable, reject, reject but mendable, etc. Finally, there is an action phase where the appropriate response is selected, eg, place the item on the 'acceptable' conveyor, send for rectification, etc. The advantage of this approach is that it serves to direct the training analyst to the area ,of research which is appropriate for a particular training problem, This model could be used in conjunction with the Hierarchical Task Analysis described earlier. Whichever technique is applied, the results of the task analysis will enable the content of the training schenie to be specified, as well as providing the training analyst with a detailed appreciation of the task.
Constituents of an inspection training programme 3. Derivation of training content This stage involves the specification of the constituents of the training programme required to meet the criteria defined
Three basic requirements can be identified as being of particular importance for effective inspection.
in (2).
(a) (b)
The attitudes of the inspector to the work. Job knowledge.
4. Designof training methods and materials
(c)
Possession of the specific skills required to perform the task.
The constituents of the programme set out at stage (3) will be translated into practical training materials at this stage. The specific techniques available will be considered in detail in later sections of this paper. 5. The training programme At this stage, the potential inspectors will encounter the training programme, Different forms of training programme may be necessary for workers of different backgrounds, eg, new employees, or existing workers being retrained. The performance of graduates from the training programme is compared with the system criteria. During this validation phase, modifications may be made to the programme in order to achieve the objectives of the system.
Task analysis procedures Task analysis is a systematic procedure which is used for defining training objectives and specifying training content. Task analysis is partly descriptive, in that it delineates the nature of the task in an orderly way, at varying levels of detail. It attempts to isolate the critical characteristics of the task for which specific training techniques will be appropriate. The prescriptive aspect of task analysis provides the basic framework for the training content. One particular form of task analysis which can be applied to inspection tasks is the Hierarchical Task Analysis of Annett and Duncan (1967). This approach successively redescribes a task at increasing levels of detail. At each level, particular aspects of the task likely to require specific training methods are isolated. Another approach, which may be regarded as being
140
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Attitudinal factors are of particular importance in inspection. Because the inspector is usually the last person to see the product before it reaches the customer, his attitude to his work wilt directly influence the quality image of the company. Unfortunately, in this country, inspection is often regarded as an unproductive, and therefore low status, job. Training, and indeed company policy as a whole, should be directed towards emphasising the importance of the inspector, whose status and remuneration should reflect this importance. There are several types of job knowledge require d for inspection work. The worker has to learn the basic protocols and procedures required to carry out the task. eg, where to find the items to be inspected, what to do when particular types of defect are encountered, and the paper work required. A different kind of job knowledge is required to recognise the different categories of defect which will occur. An appreciation of the manufacturing processes involved in producing the product and their effects on the types of defect likely to occur, is also important. There is a distinction to be made between the knowledge required for recognition and other types of job knowledge. Factual knowledge can usually be explicitly defined. On the other hand, it is often only possible to give examples of specific defects. The inspector has to learn to generalise from these examples to the types of defect actually encountered in the real inspection task. The final requirement is the possession of the specific skills required to perform the task. In the case of inspection, these are primarily the decision making and perceptual skills involved in the detection and recognition of defects.
Although a training programme will include each of the three areas discussed up to now, the remainder of this review will concentrate on the last area, the development of the specific skills required for inspection. This emphasis is a reflection of the relative amount of work which has been carried out in these areas. Very few studies exist in which attitudinal and motivational variables have been investigated. Some of these studies are discussed in Wiener (1975). On the other hand, there has been some investigation of training approaches to develop recognition skills, which will be discussed in detail in the next section. The overall emphasis of the remainder of this review will be towards training techniques applicable to inspection tasks requiring a high degree of subjective judgement of quality. The types of industrial inspection involving simple go/no-go decisions, using gauges or instruments, can usually be trained for using procedures similar to those for simple manual tasks.
Training approaches to inspection skills The following review will first consider the principles which have emerged regarding training for detection and recognition skills in general. Much of this work was carried out in the context of auditory detection in sonar applications. Nevertheless, the consensus of opinion is that these results can be readily generalised to inspection tasks. The second part of the review will describe some applied studies of inspection tasks or situations which closely resemble them. The large body of studies which analyse training for vigilance tasks will not be discussed, because it is considered that most real industrial inspection tasks are very dissimilar from the classical vigilance situation.
Knowledge of results and cuing One of the most general principles which has emerged from research in detection and recognition skills is the importance of feedback or knowledge of results (KR) in the acquisition of these skills. KR has been defined as "knowledge which an individual or group receives relating to the outcome of a response or group of responses", Annett (1961). In the context of inspection training, KR can be used in several ways. The usual method is to present the trainee with a batch of items, some of which contain characteristic defects. The trainee attempts to identify each item as defective or otherwise, and is given immediate feedback as to the correctness of his decision after each response. In some cases, he is told whether he made a correct decision or false alarm, or sometimes only information regarding correct detections is given. KR has been shown to be an essential requirement for almost all types of learning, Annett (1969). It is usually regarded as promoting learning in three ways: (a)
(b)
(c)
Its motivational effect. By providing the learner with information regarding his progress, KR encourages him to remain in the learning situation.
regarding the critical differences between defects and perfect items. Some workers have suggested that perceptual learning may occur via different mechanisms than those operating in motor learning, eg, Gibson (1953). In a series of studies, Annett and Clarkson (1964), Annett and Paterson (1966, 1967) proposed that perceptual learning took place via the simple pairing of a stimulus and its name. This implies that an overt response on the part of a trainee is not really necessary. Essentially, Annett suggested that the observer built up a mental 'template', which was an internal representation of the defining characteristics of defective items. This implied that the informational content of KR was of particular importance. Consistent with this approach, the technique of 'cuing' was advocated as an alternative to KR. Cuing involved the presentation of a series of examples of signals (defects) and non-signals (perfect items). Prior to each presentation, the trainee was told whether the next item was going to be a signal or otherwise, and no overt response was required. The warning was said to alert the learner, thus enabling him to attend more carefully to the defect characteristics. This approach seemed to be equal, or in some cases, superior in effectiveness, to conventional KR for many of the tasks considered by Annett. With later experiments, using more complex stimuli, Annett and Paterson (1967) found that cuing and KR were equivalent in their training effect, when they provided the same degree of information. This can be interpreted as providing support to the 'template' approach to perceptual learning. In practical terms, the choice between cuing and KR seems to be largely a matter of taste, although some workers have suggested that trainees can become 'cue dependent', ie, their performance deteriorates when the cue is removed. The gradual reduction of the level of cuing during training ('lading') has been used to reduce this effect.
Whole and feature-analysis approaches to perceptual training Two basic approaches to perceptual training can be identified. In the first of these, the 'whole' approach, the pattern to be recognised is used more or less intact from the beginning of training. This is the approach used in the studies described in the last section. In the alternative 'feature-analysis' approach, recognition is said to involve the synthesis of separate aspects or features of the pattern, or defect. Wallis (1963) gives a comprehensive description of an example of this approach. Wallis described his procedure as an analytic-synthetic methodology. Essentially, the salient characteristics of the defects are first learnt separately, and then brought together during training, until the signal can be recognised as a whole. The main features of Wallis's approach can be summarised as follows: 1. The emphasis is placed on realistic teaching being used. Even at the intial stages of training, the items used are real examples or exact facsimilies.
The reinforcement of observing responses. The observing response is an information seeking response on the part of the observer. This encourages him to actively examine the items being inspected.
2. Tile early stages of training attempt to distinguish between the characteristics of the non-defects and the cues characteristic of defects.
Its informational content. Many learning theorists regard this as the most important attribute of KR. KR provides information feedback to the trainee
3. Graded training exercises are given such that the cues involved in discrimination are initially very easy to discern, and are presented one at a time. Subsequently,
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the exercises are increased in difficulty until the cues are more representative of those encountered in the real task. KR is used at this stage to enahnce discrimination and to reduce false alarms. 4. An additional object of the training exercises is to accustom the learners to responding under paced conditions. Many recognition tasks allow only a limited sampling time in which to acquire the information needed to make a .discrimination (eg, paced conveyor belt inspection). 5. During the early training exercises the learner provides a verbal commentary regarding the cues being acquired, and their significance in the total recognition process. 6. Finally, the emphasis of the training changes from an analytic to a synthetic approach. Instead of responding to separate cues, the trainee has to make an overall judgement based on all the evidence available. WaUis emphasises the importance of KR in this approach, and suggests that feedback should be 'frequent, detailed and immediate'. He warns that although supplementary cues and information may be useful in the early stages of learning, they must be withdrawn later to prevent cue dependance. The validity of several of the techniques employed by Wallis is supported by other work. Gibson (1953) also found that experience with realistic material was important, and that learning could occur simply by exposure to the material. Mackie and Harabedian (1964) found transfer of training more effective when authentic material was used.
The relative effectiveness of different forms of training Gibson (1947) had found that whole methods were slightly less efficient than feature analysis approaches. However, studies by Allan (1957, 1958) comparing two training techniques for aircraft recognition, came to the opposite conclusion. Annett (1971) carried out an extensive series of experiments comparing the two approaches in a sonar recognition task. In all, 15 different training methods were compared. The overall conclusions were that there was no particular advantage in using feature analysis approaches compared with simple whole methods. Both techniques produced a substantial training effect. The most important single factor seemed to be the sheer experience of named samples of the various types of signal to be identified. The study also found that there was no particular advantage in ordering the material in terms of difficulty. With regard to the verbal explanations used by instructors, it was found that simple instructions were most effective. Also, it was best to concentrate on the one best cue which characterised the signal (defect). Detailed instructions about a variety of cues could not be dealt with effectively and could even depress performance.
Applied studies of training for inspection Although the studies considered in this section are not authentic inspection tasks, they share many of the characteristics of such tasks, eg, complex items. Learning effects in laboratory simulations of inspection tasks have taken place without any specific training being given, eg, Smith and Adams (1971) and Lion et al (1968). This tends to confirm Annett's assertion that learning can occur
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simply by exposure to the material to be learnt. In an industrial setting, Chancy and Tee1 (1967)have used several approaches in training machined parts and photomask inspectors. Their most commonly applied technique, known as job sample instruction, involves providing the inspectors with KR after inspecting test items containing typical defects. Martinek and Sadacca (1965) report a study in training photo-interpreters to identify targets on aerial photographs. Martinek provided the photo-interpreters with an error key which analysed the commonly occurring errors of previous interpretations. This produced fewer errors of commission than a key in which the the characteristic features of the various target types were set out. The error key can be regarded as a form of group KR. Another photo-interpretation study, Cockrell and Sadacca (1971) showed that KR was more effective in a team context when team members first scanned a photograph independently and then discussed the results together immediately afterwards. The greatest gains in proficiency were made by the least able members of the team. Although detection performance was improved significantly, there were greater gains in reducing the number of misidentifications and false alarms. Powers et al (1973) attempted to improve performance in this task by modifying the interpreters' scanning strategies. Structured search practice increased the number of target detections at the expense of a greater number of false alarms. A 'speed reading' training technique, designed to reduce fixation times and expand the visual field, halved the search time. but did not produce any inc tease in accuracy. Training using the error key approach discussed earlier significantly reduced false alarms. Brock. Wells, and Abrams (1974) provided a complete training programme for the examiners of radiographs used in non-destructive testing. The programme consisted of tape/slide presentations which gave examples of various types of defect, and then a KR phase where radiographs containing defects were presented and the student given full KR after he had attempted to identify them. A selfadministered test then determined if the student should go on to the next module or re-take prevtous modules. There was a gradual increase in difficulty of the radiographs both within and between modules. The programme was highly successful, in that it reduced the time needed to train to the required criterion from an average to 80 h to 10-9 h.
Research on inspection training: general conclusions It is difficult to evaluate the practical applicability of much of the research discussed in earlier sections because of the very limited number of studies which have used authentic industrial inspection tasks, Nevertheless, the applied studies which have been carried out have tended to confirm the laboratory based findings. Most of the studies agree on the importance of allowing the trainee to build up a 'mental model' of the characteristics of good and defective product. This seems to be achieved by extensive experience with authentic examples of defects and non-defects. Both cuing and KR seem equally effective in this. Similarly. both whole and part approaches appear to promote equal degrees of learning. Where the task being trained for is paced, this should be explicitly considered in the training programme.
The instructional style to be used during training is also important. Annett's findings that simple instructions, dealing with one cue at a time, were most effective seem relevant here. Virtually no formal work has been carried out on the long term retention of inspection skills, although Thomas {1962) discussed the importance of recalibrating the inspector to reduce the possibility of drifts in standards over time. The most effective form of this type of retraining needs to be established. Nearly all the training techniques discussed up to now have been designed to enhance the inspector's sensitivity(d') in Signal Detection Theory terms. Although sensitivity is probably the most important factor, there remains the possibility that the inspector might be trained to employ the most efficient rejection criterion. Signal Detection Theory suggests that this is defined by the probability of a detect occurring, and the relative costs and values of rejecting good product or missing defects. Embrey (1976) carried out some preliminary experiments which suggested that some forms of training do enable the inspector to adjust the criterion more effectively in accordance with changing defect percentages. This work requires elaboration before being included in inspection training.
Conclusions The consideration of a large body of research has produced some practical guidelines for training inspectors. These need to be implemented within a systematic procedure for the development of an inspection training scheme. The procedures of Eckstrand, set out in the early sections of this paper, can be used for this purpose. The importance of a systematic task analysis, to enable the techniques appropriate for a particular situation to be used, needs to be re-emphasised. Finally, it is hoped that in the future the ergonomist will be involved in the design of quantity control systems from the outset, in order to apply ergonomics principles in the most cost-effective way.
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Powers, J.R., Brainard, R.W., Abram, R.E,, and Sadaeea, R. 1973 Training techniques for rapid target detection. US Army Research Institute for the Behavioral and Social Sciences, Technical Paper 242~
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Thomas, L.F. 1962 Ergonomics, 5, 429-434. Perceptual organisation in industrial inspectors.
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