The War on Error – Vol. II: Poka Yoke (What Is and Is Not)

     Of the “eight wastes of lean,” the impacts of defects may be the easiest to understand.  Most find the need to rework or replace a defective part or repeat a faulty service, and the subsequent costs, to be intuitive.  The consequences of excess inventory, motion, or transportation, however, may require a deeper understanding of operations management to fully appreciate.
     Conceptually, poka yoke (poh-kah yoh-keh) is one of the simplest lean tools; at least it was at its inception.  Over time, use of the term has morphed and expanded, increasing misuse and confusion.  The desire to appear enlightened and lean has led many to misappropriate the term, applying it to any mechanism used, or attempt made, to reduce defects.  Poka yoke is often conflated with other process control mechanisms, including engineering controls and management controls.
     To effectively reduce the occurrence of errors and resultant defects, it is imperative that process managers differentiate between poka yoke devices, engineering controls, and management controls.  Understanding the capabilities and limitations of each allows appropriate actions to be taken to optimize the performance of any process.

Poka Yoke
     Poka yoke is a Japanese term; the commonly accepted translation is “avoid mistakes” (yokeru ≈ avoid; poka ≈ mistake), or “mistake-proofing” in a production context.  Poka yoke devices come in two forms:
(1) a device, such as a fixture or tool, that interfaces with a part in such a way that prevents an error from occurring.  Example:  a nearly-symmetrical part is drilled in an off-center location; incorrect orientation of the part when drilled will render it scrap.  To prevent this easily-committed error, a fixture is used to engage the asymmetric features, properly positioning the part for the drilling operation.  Operators are no longer required to closely inspect the part, increasing productivity of the operation, while eliminating scrap due to incorrect orientation of the part.  This is called the contact method; see Exhibit 1 for a visual representation of this example.

Exhibit 1:  Contact Method Poka Yoke Device

(2) a device that makes an error or defect obvious.  Example:  each product to be assembled is built from a kit containing every component required; nothing extraneous is allowed in the kit.  Before the product leaves the assembly station, a glance at the kitting tray confirms that it is empty and, therefore, no components are missing from the assembly.  If a component remains in the kit, the error can be corrected immediately, before it affects downstream processes or the customer experience.  This is a fixed-value type poka yoke; see Exhibit 2 for an example.

     Poka yoke devices can protect processes from various types of errors (see Vol. I for a brief introduction to error types).  Many inadvertent errors caused by the pressures of production requirements (rate), distraction, fatigue, or other factors can be prevented by using poka yokes.  An operator’s lack of experience or forgetfulness will have less impact on outcomes when poka yoke devices are in use.  Identification errors can be eliminated by using a device that amplifies the differences between similar components (see Exhibit 1 example).  In highly-contentious environments, the ability of poka yoke devices to make willful and “intentional” errors (see comments on sabotage in Vol. I) more difficult to commit adds greatly to their value.
     Operator 100% inspection is prohibitively expensive for most operations, particularly high-volume production.  Poka yoke devices, on the other hand, are typically inexpensive (and a non-recurring expense), making cost-effective 100% inspection attainable.
     Poka yokes are often completely passive, drawing little attention from operators.  Any degradation in performance that may occur is easily overlooked.  For this reason, all poka yoke devices must be included in equipment maintenance plans and verified periodically.  Poka yoke verification with “limit samples” or other test pieces is typically done at every startup and shift change.  If a device accepts an incorrect part or rejects a correct part, proper function must be restored before the process is allowed to run.  For example, a fixture that has worn to the point that it accepts oversize parts, or no longer holds parts securely, should be replaced before any more parts are processed.
     The purest form of poka yoke is a simple, mechanical fixture – no moving parts, no electronics – in which correct parts can be placed and incorrect parts cannot.  As features are added, simplicity is sacrificed and, eventually, the term “poka yoke” becomes “fuzzy” and confusing, as discussed in the introduction.  To minimize confusion caused by fuzziness, we will now turn to descriptions of the other mechanisms of error reduction mentioned:  engineering controls and management controls.
 
Engineering Controls
     As the performance and reliability of engineering controls improved, it became easier to conflate them with poka yoke devices.  High reliability and consistent performance allows these systems to fade from the consciousness of operators and managers alike.  It may be easy to do, but this class of controls should not be forgotten or ignored.  Unlike poka yoke, most engineering controls are active devices; they are components of equipment and systems that require regular attention to remain safe and productive.
     Many engineering controls in common use are operated via PLCs (programmable logic controllers), though this is not exclusively the case.  PLC-operated controls may include proximity sensors, limit (travel) switched, pressure switches, encoders, force transducers, or other instruments.  PLC controls can also be safety devices, such as light curtains or two-hand anti-tie-down activation switches.  The program run by the PLC is also an engineering control.
      Whenever possible, engineering controls must be designed to “fail safe.”  That is, failure of a control to function properly must prevent further operation of equipment or processing of parts.  Doing so eliminates consumer risk (Type II) by ensuring that parts cannot be produced without completing all quality verifications.  Producer risk (Type I) is also kept low; production interruptions receive prompt attention to identify and correct malfunctions.
     Where fail-safe operation is not possible, frequent verification of proper function is required.  Clearly, fail-safe operation is preferred; achieving it requires equipment designed to support the initiative, selection of appropriate instruments (sensors, switches, etc.), and proper programming.
     Non-PLC engineering controls include masking of parts prior to painting or plating, chemical “recipes,” process control limits, special work instructions, technical standards, and special handling equipment used when manual handling could affect product quality (e.g. contamination).  Fail-safe operation of these controls may not always be feasible, but may be possible for some operations if used in conjunction with sensors or other PLC-operated controls.
     Engineering controls generally provide less certainty than poka yoke devices.  For example, a sensor used to verify the presence and proper position of a part before initiating a machine cycle could become dirty, causing it to always allow the machine to cycle.  Defective product, equipment damage, and injury to personnel could result.  This is why sensor selection, proper programming, and functional verifications are so important.  It also reinforces the need to understand the difference between a poka yoke and an engineering control.
 
Management Controls
     Management controls consist, in large part, of policies, procedures, and instructions.  Work instructions – engineering controls – are focused on the operation of a specific machine or process, while instructions, as management controls, are broadly applicable.  For example, instructions may define a standard format of time and production reporting to be used by all employees for all equipment and processes.
     A common example of a procedure used for management control is that to be followed when defective product is found.  It could include steps needed to contain and replace defective goods and to identify and correct the source of the defect.  A number of procedures are also used to meet environmental, safety, and other regulations.
     Policies are typically published by Human Resources to define the company’s expectations in legal and ethical matters and to assure employees of fair treatment.  Acceptable behavior in financial transactions, interpersonal relationships, and interactions with customers, suppliers, and public officials are examples of topics typically addressed in a company’s policies.
     Other management controls include production quotas, information systems access, assignment of authority (to halt production, initiate disciplinary action, etc.), and training (development plans and instruction).
 
In the Service Sector
     The previous discussion and examples focused on discrete manufacturing operations.  Many of the same concerns expressed also exist in process and service industries.  The misappropriation of the term poka yoke seems especially egregious in the service sector.
     The presence of the customer in service operations increases the potential for errors – either customer or provider can make a mistake that effects service delivery.  The variability that customers introduce also makes the development of poka yoke devices difficult, if not impossible.  Poka yokes are designed around a small number of known variations; services are often much more varied and unpredictable.
     The wide range of services available also limits the application of technology-based engineering controls.  The more personalized the service, the less likely that sophisticated technology can be used to prevent errors.  Applications are often limited to “back office” operations; that is, routine steps performed in the customer’s absence.  Nontechnology-based engineering controls, such as process control limits and work instructions, are likely to be much more prevalent in service operations.
     Given the challenges discussed above, it will likely come as no surprise that providers rely heavily on management controls to maintain service quality.  Broadly-applicable procedures for customer engagement are necessary to provide a consistent experience for customers with very different needs.  It is highly-developed and well-executed management controls that are primarily responsible for the ability to deliver consistent high-quality personal service.
     A benchmark for consistency, fast food restaurants depend on management controls for the customer experience (order entry, wrapping and presentation, etc.), but also employ significant engineering controls behind the scenes.  From temperature controllers and timers to the layout of the counter and drink station, engineering controls contribute greatly to providing a consistent customer experience in every location.
     Poka yoke devices exist in the fast food environment (e.g. egg ring), but are much less common than other types of error-reduction mechanisms.  Impressive levels of consistency and misunderstanding of the term lead some to identify service providers’ error-reduction efforts as poka yokes.  It is a testament to their effectiveness, but it is mostly incorrect.
 
Hierarchy of Controls
     All of the error-reduction mechanisms discussed support a “no fault forward” philosophy.  They are complementary approaches; there is, however, an implied hierarchy.  Prevention of errors is strongly preferred to other error-reduction schemes, followed by “capture at the source.”   If neither of these can be achieved, the sooner an error or defect is detected, the better.  The worst case is for a customer to receive a faulty product or unsatisfactory service.  This clearly places poka yoke devices atop of the hierarchy.  Engineering controls can be used to prevent or detect errors and defects, positioning them next in the hierarchy.  Management controls are at the bottom of the hierarchy; this is where responses to failures of higher-level controls are typically defined.  They rely exclusively on human intervention and, thus, are less reliable.
     An alternate formulation of this hierarchy considers the target of each control mechanism.  Poka yoke devices target part control, ensuring that every part processed is acceptable.  Engineering controls are used for process control, ensuring that each iteration of a process is performed consistently across time, parts, and operators.  Management controls target consistent behavior of people across time and circumstances.
     The hierarchy also represents relative cost-effectiveness typical of the control mechanism categories.  Poka yoke devices are typically low-cost and highly effective.  Management controls are of moderate cost, but their effectiveness is dependent upon an organization’s culture and the quality of the controls developed.  Engineering controls occupy the middle ground; they can be extremely effective, but can also become quite costly.  Technology solutions, in particular, must be well-designed for their purpose to maintain both affordability and effectivity.
 
     Being “tough on the process, easy on the people” signals to employees that they are valued in ways that platitudes (“our people are our most valuable resources”) simply cannot communicate.  An organization’s commitment to developing and maintaining appropriate error-reduction mechanisms communicates standards, values, and expectations more clearly than any speech or memo ever will.
 
     For additional assistance with error reduction, feel free to ask a question in the comments section below or contact JayWink Solutions for guidance customized to your specific situation.
 
     For a directory of “The War on Error” volumes on “The Third Degree,” see “Vol. I:  Welcome to the Army.”
 
References
[Link] “Quality and Productivity Improvement:  A Study of Variation and Defects in Manufacturing.”  Edgardo J. Escalante; Quality Engineering, March 1999.
[Link] “What is Mistake Proofing?”  American Society for Quality.
[Link] “Poka-yoke.”  Wikipedia.
[Link] “What is the Poka Yoke Technique?”  Kanbanize.
[Link] “Factsheet:  “‘Poka Yoke’ error proofing.”  Chris Turley, LeanQCD.com.
[Link] “Poka-Yoke.”  iSixSigma
[Link] “Error Proof the Pokayoke to Build in Quality.”  Jon Miller, Gemba Academy; July 26, 2006.
[Link] “Poka Yoke in the service sector.”  Chris Turley, LeanQCD.com.
[Link] “Poka-Yoke.”  Reference for Business.
[Link] “A Rose by Any Other Name…”  Glenn Nausley, Motorized Vehicle Manufacturing; June 20, 2018.
[Link] “Poka-Yoke is Not a Joke.”  Michael Schrage, Harvard Business Review; February 4, 2010.
[Link] The New Lean pocket Guide XL.  Don Tapping; MCS Media, Inc., 2006.
[Link] The Lean 3P Advantage.  Allan R. Coletta; CRC Press, 2012.
 
Jody W. Phelps, MSc, PMP®, MBA
Principal Consultant
JayWink Solutions, LLC
jody@jaywink.com