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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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