چارچوبی برای ارزیابی دستگاه های poka-yoke

This study introduces a framework for assessing poka-yoke devices (PD), encompassing both those designed for quality control (referred to as quality PD) and those designed to control hazards to health and safety at work (referred to as safety PD). The framework assesses the processes of the design, operation and maintenance of PD, rather than the outcomes of these processes. The development of the framework involved three stages: (a) defining 15 attributes of PD, identifying those that provide fail-safe characteristics and those that inform best practices in design, implementation and maintenance; (b) defining what the set of evidence and what the sources of evidence should be for assessing the existence of each attribute; the sources include documents, interviews, observations and a meeting to discuss the results of the assessment with company representatives; and (c) defining a scoring system to express the results of the assessment. The application of the framework is illustrated by means of assessing four PD; two of them being concerned with quality and two with safety.

The principles and practices of lean production (LP) have been increasingly used by a number of industries [1] and [2]. This study emphasizes the use of poka-yoke devices (PD), a LP practice especially tailored to eliminate the production of defective parts, complementing statistical process control techniques [3] and [4]. Since this term started being disseminated in the West in the context of spreading the Toyota Production System [5], PD has been a topic of interest, mostly among practitioners, due to the apparent simplicity of how they function and their intuitive design features [6]. PD have been used in a variety of contexts, such as construction, health care and information technology, but not necessarily associated with LP implementations [7], [8] and [9]. However, there seems to be a scant academic interest in this topic, conveying the misleading message that the design and operation of PD can be explained by common sense [6]. In fact, similarly with other important LP practices (e.g., kaizen and visual management), the literature on PD is, to a great extent, directed to practitioners [10]. As a result, the attributes of PD are presented in a fragmented way over a number of studies [5], [7], [9] and [11], without being integrated into scientifically validated frameworks for designing and assessing them. In this study, the focus is on the development of a framework for assessing the use of PD. No reference was found on methods, which had been tested and validated in real-world settings, which could be used for assessing the use of PD. From both a practical and theoretical perspective, the need to develop a framework for assessing PD is part of the need to develop methods for assessing the use of lean practices. In fact, compared with the efforts made to address “how to become leaner”, the issue of “how lean the system is” has received less attention [12]. Although a number of methods have been drawn up to assess the extent to which a company adopts lean principles [12], [13] and [14], fewer methods [15] and [16] aim at assessing the use of lean practices, the implementation of which takes place directly and visibly on the shop-floor (e.g., PD). Survey has been the preferred strategy for assessing the use of lean practices, aiming at investigating large samples of companies [16], [17], [18], [19] and [20]. While these surveys have merits, they tend to be of little use as tools to companies that aim at assessing how their particular lean practices are performing. Thus, similarly with previous studies on the assessment of lean practices [15], this study adopts the method of auditing as an alternative. This method is well-known in most industries, combining both qualitative and quantitative assessments [21], which can provide more pragmatic insights for individual companies, in comparison with those provided by surveys. An audit is a systematic, independent and documented process to obtain evidence and to assess them objectively, measuring the extent to which audit criteria are met. Auditing allows the gap between desired and actual performance to be identified, as well as to enable the assessment of information that can be used to improve performance [22]. The attributes encompassed by the framework proposed by this article are applicable both to PD designed for quality control and to those designed to control hazards to health and safety at work (H&S). This integrative perspective between quality and H&S might be considered as a distinctive feature of the proposed framework, as both companies and the academic literature usually deal with each type of PD separately. According to Stewart and Grout [6], this is due to the fact that, typically, researchers and practitioners devoted to quality control have little technical knowledge of H&S, and vice versa. The application of the framework is exemplified by means of applying it so as to analyze four PD: two of which are concerned with quality and the other two with safety.

This study introduced a framework for assessing the processes of design, operation and maintenance of PD, which has three distinctive characteristics: (a) definition of attributes applicable both to quality and safety PD; (b) definition of attributes that provide fail-safe features and attributes that are simply best practices of design, operation and maintenance; and (c) a scoring system, which supports the comparison among PD. Also, criteria for defining the attributes were stated in order to support adaptations of the framework to the evolution of PD practice and theory. These criteria add conceptual precision and generalizability to the framework, since possible adaptations will be guided by properly defined principles. The framework might be used at least during two stages of the life-cycle of a poka-yoke: (a) during the design of a new poka-yoke, as a check-list and source of insights for designers; (b) as a tool to be applied on a regular basis to all PD of a company, thus providing benchmark measures. In this respect, the frequency of internal audits could be based on the frequency of external audits, which are usually mandatory by clients and have well defined timeframes (e.g., annual). Moreover, as it was pointed out by the assessment of PS2 in the construction company, the framework is also useful for companies that do not use lean production. Even though it is a single case study, it has also shown that the results of the assessment can be better in a non-lean environment. For example, PS2 performed better than PQ2, which was used in a major transnational that formally adopted lean production. As an advantage, PS2 was implemented as a requirement of regulations, having been used for many years in a fairly uniform way by construction companies all over the country. By contrast, PQ2 was designed as a prototype. Of course, an application of the framework in a larger number of PD and companies is necessary to identify trends concerning lean versus non-lean environments. A hypothesis to be tested, based on the preliminary piece of evidence provided by this study, is that, for a safety poka-yoke, the implementation within a lean environment is less important, since they are often demanded by regulations, rather than due to the specific production philosophy adopted by the company. The comparison between quality and safety PD also indicated that the companies were not aware of their similarities and, as a result, they were not taking advantage of the synergistic relationships. In company A, it was clear that quality and safety PD were managed by completely different mechanisms and staff. In this company, no reasonable justification was identified for not extending some practices used by quality PD to the safety ones. As another example of the impact of this fragmented management structure, PQ2 displayed H&S hazards, a problem originated from the lack of a design team involving both H&S and quality control staff. The case studies have also shown that the auditors should be acquainted with the concepts underlying the attributes of PD, which require background in both lean and human factors. This required knowledge base is a result of the socio-technical nature of the poka-yoke design process, which emphasizes the need to go beyond the traditionally fragmented way that, companies, and often operations management research, deal with human and technical issues in the work environment. In addition to the expertise required, another possible drawback to applying the framework is concerned with the unavailability, in the companies investigated, of the recommended sources of evidence. For example, in the assessment of PQ1 and PQ2, there was no preliminary hazard analysis of the operations, making it difficult to assess the extent of the H&S hazards. Nevertheless, the unavailability of the sources of evidence should be interpreted as a result of practical implications for the company investigated, since this indicates the opportunity for providing sources. Another important insight arising from the case studies concerns the risk of being too reductionist when applying the framework. The PD is frequently a small component of a broader workstation. Therefore, sometimes it is not straightforward to define the boundaries of the poka-yoke (i.e. where the poka-yoke ends and where the rest of the workstation begins). This difficulty was encountered, for example, during the assessment of PQ1. The lack of calibration of the screwdriver, while being a problem, was not considered to be a failure of the poka-yoke, since the screwdriver, by itself, had not any inspection role concerning the abnormality investigated. Nevertheless, a holistic view during the application process made it possible to identify this shortcoming, even though it did not alter the score of the poka-yoke. By contrast, in the assessment of PQ2, the whole workstation was considered to be the poka-yoke. On the one hand, this eliminated doubts on the border between the poka-yoke and the workstation. On the other hand, this was intrinsically bad, since, if the workstation was the poka-yoke, this meant that it was fully dedicated to reactive inspection. As regards the scoring system, it proved to have discriminatory power, according to the company representatives who received feedback of the results. However, two important recommendations should be stressed to reduce the subjectivity of the assignment of scores: (a) at least two auditors should reach a consensus on the final score, as happened in the case studies; (b) a report should justify the scores assigned, especially those that were not equal to the pre-defined performance levels proposed by the framework. In other words, this means that the auditors should leave a trail, to show how they reached their scores and conclusions from the data available. As a consequence of the limitations of this study, a number of opportunities to continue this research in future projects were detected, such as: (a) adapting the framework to other sectors, such as services (e.g. hospitals) and transportation (e.g. aviation and railways), since changes in the attributes might be necessary; (b) adapting the framework so it can be used in other stages of the life-cycle of a product, rather than in manufacturing (e.g. design and maintenance); (c) investigating how the framework could be used in the design stage of new PD; (d) assessing if there are statistical correlations between the level of compliance with the attributes and other factors, such as the technological complexity of poka-yoke; the sector in which they have been used; and the extent to which LP has been used in the organization; and (e) using the framework to identify, among the existing publications that catalog examples of PD, those which are in compliance with the attributes that provide fail-safe characteristics and, as a result, should be of primary interest with regard to disseminating them. Two additional needs to be addressed by future studies are particularly important since they have to do with methodological issues associated with the framework: (a) assessing its reliability (i.e., checking if similar results will be found by different teams of auditors); and (b) assessing its ability to predict the effectiveness of PD (i.e. the predictive validity of the framework), in terms of measures such as scrap rates, productivity, client satisfaction and incident rates.