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‘POKA YOKE OR QUALITY BY MISTAKE PROOFING DESIGN AND CONSTRUCTION SYSTEMS Iris D. Tommelein1 ABSTRACT The Japanese concept ‘poka yoke’, translated into English as ‘mistake proofing,’ has been mentioned at previous IGLC conferences. This notwithstanding, mistake proofing appears to not have been (nor be) systematically researched or practiced in the lean construction community. To raise awareness of opportunities provided by thinking with mistake proofing in mind as a means to build quality into project delivery, this paper summarizes the philosophy that underlies mistake proofing. Examples illustrate how mistake proofing applies to the work done within one specialty trade, how manufacturers and fabricators can design their products so they cannot be constructed defectively, and how architects and engineers may conceive of system designs that are less likely to fail during construction or in a product’s life cycle. Reader contributions to an online repository of mistake proofing applications in the architecture-engineering-construction (AEC) industry, posted at http://p2sl.berkeley.edu/pokayoke/, will be gratefully acknowledged. KEY WORDS poka yoke, mistake proofing, jidoka, autonomation, design, engineering, system, specification, construction, safety, quality, constructability, tolerance management, life-cycle performance, lean construction translated as ‘autonomation’ in INTRODUCTION AND English, as together they form a pillar DEFINITION of the Toyota Production System. Shingo (1986), a master mind of the Autonomation refers to machines Toyota Production System, introduced built to detect problems and stop by the concept of ‘poka yoke’ in themselves, so as to “relieve the Japanese, translated as ‘mistake burden of constantly supervising a proofing’ in English, in his book titled machine, and allow [people] to use Zero Quality Control: Source their talents for more beneficial things Inspection and the Poka-yoke System. (like adding value)” (Liker and Meier This concept goes hand-in-hand with 2006 p. 177) “The purpose of the concept of ‘jidoka’ in Japanese, autonomation is the rapid or immediate address, identification and correction 1 Director, Project Production Systems Laboratory, http://p2sl.berkeley.edu/, and Professor, Engineering and Project Management Program, Civil and Environmental Engineering Department, 215-A McLaughlin Hall, University of California, Berkeley, CA 94720-1712, Phone +1 510/643- 8678, FAX +1 510/643-8919, tommelein@ce.berkeley.edu 195 ‘Poka Yoke’ or Quality by Mistake Proofing Design and Construction Systems Iris D. Tommelein of mistakes that occur in a process… proofing examples. The aim of this Once the line is stopped, a supervisor effort is to develop a community or person designated to help correct knowledge base and to spur discussion problems give immediate attention to around mistake proofing opportunities the problem the worker or machine has in the architecture-engineering- discovered. To complete jidoka, not construction (AEC) industry. only is the defect corrected in the product where discovered, but the APPLICABILITY OF MISTAKE process is evaluated and changed to PROOFING IN THE AEC remove the possibility of making the INDUSTRY same mistake again. This ‘mistake- Shingo’s premise of ‘zero quality proofing’ of the production line is control’ is to ‘do it right the first time.’ called poka yoke.” (Superfactory Bodek stressed this idea in his preface 2008). to Shingo’s book (1986 p. vii) by Many online glossaries with lean stating that we should “drop the idea production terms include ‘poka yoke’ that defects are a normal part of (e.g., http://www.nummi.com/tps.php) manufacturing.” In the AEC industry, and at least one website has been this thinking is contrary to the reliance dedicated to this topic of practitioners on inspection and (http://www.mistakeproofing.com/). punch lists as means to work towards Books have been written on the an acceptable end product, hopefully application of mistake proofing in one that is satisfactory and of quality! specific industries (e.g., Grout 2007 is To eliminate the need for quality on health care processes). Lean control, the practice of mistake construction researchers have proofing sets out to prevent errors or mentioned the concept for many years defects from occurring in the first at previous IGLC conferences and place. elsewhere (e.g., Koskela 1992, dos Mistake proofing is particularly Santos et al. 1998, 1999, dos Santos well suited for the AEC industry with and Powell 1999, Moser and dos its low-volume and mixed production Santos 2003, Adbelhamid and Salem systems where statistical quality 2005). This notwithstanding, the control methods cannot be practice of mistake proofing still implemented due to lack of data and appears to not be systematically un-timeliness of findings that result pursued by researchers and from after-the-fact data processing. practitioners in the lean construction Mistake proofing requires a different community. To raise awareness of way of thinking about production opportunities provided by thinking processes and its constituent with mistake proofing in mind as a operations, but once practitioners have means to build quality into project learned to recognize mistake proofing delivery, this paper summarizes the devices, their new mind-set will enable philosophy underlying mistake them to spot numerous opportunities proofing, illustrates opportunities for available to mistake proof their application of this concept in practice workplace. They will find that many by means of examples, and solicits mistake proofing practices can be contributions from readers who may implemented at a minimal cost, though wish to volunteer other mistake th Proceedings for the 16 Annual Conference of the International Group for Lean Construction Production System Design 196 ‘Poka Yoke’ or Quality by Mistake Proofing Design and Construction Systems Iris D. Tommelein some do require investment in new and-by itself yields advantages, product development. practitioners will reap the greatest Mistake proofing could be thought benefits from mistake proofing when of as a practice that is part of pursuing applying it in concert with other lean constructability, that is, changing a practices. design so that it could be built ‘better’ Shingo (1986 p. 135) “thought that (e.g., more easily, cost effectively, explaining poka-yoke methods by safely, so it will last longer, etc.), but it means of examples would be differs from constructability in two extremely effective when it came to regards. First, the goal of mistake actually adopting the poka-yoke proofing is to improve production system” and he goes on to present system performance by eliminating numerous examples (ibid pp. 139- waste, e.g., avoiding product and 261). Likewise, this paper includes a process defects, reducing variation, selection from nearly hundred AEC and not tolerating poor quality. examples I have collected to date, to Second, efforts at mistake proofing do show the broad applicability of not necessarily coincide with the mistake proofing in various phases of timing of constructability review in a project delivery. Tommelein and Grout project’s delivery process. Simply put, (2008) describe and analyse many pursuing constructability sometimes more examples and offer more detail means cutting costs after a design than is presented here. Examples in already has been substantially this work are not intended to be developed but exceeds budget. In endorsements of the products they contrast, examples in this paper refer to. illustrate that mistake proofing is a practice for all project participants MISTAKE PROOFING IN (designers, manufacturers, fabricators, DESIGN, CONSTRUCTION, OR builders, and others) to pursue in their MAINTENANCE day-to-day work and throughout Mistake proofing applies to work done project delivery. by a single specialist or by several The purpose of this paper is to specialists. In example 1 (Figures 1 raise awareness of how mistake and 2), specialists in design and in proofing practices support lean construction have color coded implementation, specifically on distinctions that matter for their products and processes in the AEC specific work and phase of a project so industry. Mistake proofing practices as to avoid mix-ups. In examples 2, 3, contribute to improving a system’s and 4 (Figures 3, 4, 5,and 6), work has performance, for example, by reducing been ‘productized.’ Manufacturers the time it takes to perform a task have made devices to address a while also narrowing the variation of specific need and thereby reduced the that task’s duration, by making sure amount of work, and simplified the hand-offs from one task to the next are nature of the work required of field sound (not defective), and by reducing personnel. In example 4 (Figures 5 and variation in products and process 6), a component is added to the system outcomes. Though the focus in this in order to fail safe maintenance work. paper is on mistake proofing and These devices literally or figuratively though application of this concept in- turn work into ‘plug-and-play.’ th Proceedings for the 16 Annual Conference of the International Group for Lean Construction Production System Design 197 ‘Poka Yoke’ or Quality by Mistake Proofing Design and Construction Systems Iris D. Tommelein Figure 1: Color coding of design drawing shows Figure 2: Color coding shows locations for sheet different wall types for drywall cost estimate metal straps and pipe hangers on metal decking (Source: DPR, Inc., Camino Medical Project) (Source: John Mack, Southland Industries, Inc., presentation at 2007 Annual Conference of the Lean Construction Institute, San Francisco, CA) EXAMPLE 1: COLOURCODING TO hose that solves a typical fit-up IMPROVEIDENTIFICATION problem. At one end, the toilet bowl Figure 1 shows colour codes a (commode) is seated on waste-water construction estimator has assigned to pipe located in the floor, and the water distinguish various wall types. This tank rests on and connects to that base helps in clarifying and categorizing the of the fixture. At the other end, the design requirements specified by the water supply pipe runs in the wall and architects, in performing a quantity stubs out of it, ending with a valve. take off and preparing a cost estimate, The challenge is to connect the pipe at and in planning the work. Figure 2 this valve to the entry into the water shows colour codes being used on site, tank, recognizing that all construction to highlight which metal-decking work that precedes this connection step inserts belong to which trade. This is subject to dimensional variation helps, among other things, in making it (tolerances), that is, things do not get easy to assess whether or not all inserts physically located exactly where are in place prior to casting the drawings or computer models showed concrete slab on this decking. These them to be. Rather than requiring two examples illustrate mistake bending of more rigid tubing or pipe proofing approaches that help reduce and cutting it to size, a flexible hose of the likelihood of occurrence—though approximate (standard) size suits this not 100% prevention—of mistakes. purpose without requiring accurate measurement. EXAMPLE 2: FLEXIBLE CONNECTION Mistake proofing devices to TOACCOMMODATE DIMENSIONAL accommodate the manifestation of VARIATION uncertainty in physical geometry Figure 3 shows plumbing where the (dimensions and location), and mistake proofing device is a flexible accumulation of that uncertainty as work progresses, similarly exist in th Proceedings for the 16 Annual Conference of the International Group for Lean Construction Production System Design 198
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