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Eliminating Errors in Your Workplace

How to carry out a Poka Yoke exercise

Imagine a work place where errors simply do not and cannot happen! Impossible? Well probably, however that is the ultimate aim of error proofing or in Japanese Poka Yoke. This maybe a utopian ideal however there are huge improvements that can be gained without even approaching this ideal.

The following article sets out what error proofing is and how to undertake an error proofing improvement project.

The classic model for building quality into your products and services has been around a long time before the start of the industrial revolution and is known as the inspection model. See figure one.

Fig 1. Traditional Inspection Model

In essence the inspection model takes outputs from a process, inspects them through a separate process to a defined specification or standard and then either passes or fails them according to said standard. The good units can then be moved to the next process/customer whilst the failed units must then be reworked or scrapped. In a good inspection system there is also a feedback loop to the process to change or adjust the process in order to improve the quality of the output.

In principle this is an excellent process it provides a filter separating good and bad units it has a corrective action procedure and it has a feedback mechanism for continuous improvement. In time, theoretically, this process should achieve perfect products or services.

In practice however, the system has a number of short falls. The first is that the inspection process itself may be subject to failure and therefore does not protect the customer from receiving bad items. Another fault inherent in the inspection process is, that by the time the parts reach inspection some of them may already be faulty and therefore time, effort and expense have already been wasted and further cost has to be incurred in inspecting, reworking or scraping the items. A third issue with the inspection model is that in the real world the process for delivering the parts or services and the inspection process itself with be subject to variations such as different materials, people, environmental condition etc, making the task of the continuous improvement feedback mechanism much more difficult and often never achieving its ultimate aim of stemming or eliminating the errors in the process.

So, what can be done? Well about 50 years ago, a man called Shigeo Shingo working as a consultant to the Toyota motor company developed an alternative quality model which he called Poka Yoke and hailed as a "revolution in manufacturing systems". Translated into English Poka Yoke means error proofing and Shingo's model is set out in figure 2 below.

Fig 2. Error Proofing Quality Model

The error proofing quality model essentially inserts up to 3 additional checks within the production or service process before products or services even get to the quality inspection process. These are termed; source inspection, self inspection and successive inspection.

Source inspection is the ultimate quality control system. Its aim is to prevent any errors being produced at source before the process even starts. In essence a faulty, good or service or component thereof can be detected and the process fails to start. A simpler example of this in a manufacturing environment would be a go no go gauge before the start of a process which could not be tampered with or bypassed. An example in a service environment could be a rigorous check list of all the information required before that service could commence.

If a source inspection process is not possible, the next level or error proofing is self inspection. Self inspection is where the process can detect if an error has been made and can either correct the error, remove it or provide a signal for intervention. Ideally self inspections are also error proofed so that they cannot go wrong. If source or self-inspection are not possible then the Poka Yoke successive inspection model can be used. This uses successive steps in a multi step process to check the work incoming from the previous process step. This check could itself be either; source or a self inspection process.

The Poka Yoke error proofing model, in the best case scenario eliminates an error occurring before it even starts and in the worst case the product or service will have under gone a series of in process step by step inspections.

Running an Error Proofing Workshop

So having established the Poka Yoke model, how do we go about undertaking an error proofing improvement exercise? In fact we can use any existing process improvement model to undertake a Poka Yoke exercise. We could use PDCA (Plan, Do, Check, Act) within a Kaizen/continuous improvement team event or a project. We could also use DEMAIC (Define, Measure, Analyse, Improve, Control) and other methods providing they are systematic and rigorous.

Let us take a team based DEMAIC model and explain how this can be used to undertake an error proofing project.

During the Define phase the usual project charter, stake holder analysis and understanding the voice of the customers are still relevant. The team would decide what their problem was, their objective, the scope of the project and any constraints. They would then set out a time scale and expected benefits. The stake holders would be identified and managed and the requirements of the 'customer' internal or external would be understood and taken into account in the project charter.

The Measure phase would include measuring the current process performance this could include the current level of defects, rework, customer complaints, and warranty claims. You would need to make sure this information is as accurate and reliable as possible and that you could repeat this information after you've made improvements.

In the Measure phase of an error proofing exercise the team needs to map or study the process in sufficient detail to identify the sources of error. This could be achieved by walking the process and then brainstorming potential causes and select the key areas. An FMEA (Failure Mode and Effects Analysis) could be utilised to help identify and prioritise key sources of error.

In the Improve phase the team would brainstorm or generate ideas to error proof each key source. For each idea listed state whether it is a source, self or successive Poka Yoke solution. The next step is to select the best idea for each error proof and decide how this can be best tested for effectiveness. This may be for example; building a prototype and running trials to see if it is effective.

Once the error proofing solution has been proven, confirm this through collecting data and comparing it with the original process performance. If you are satisfied with this then move to the Control phase of DEMAIC. The aim here is to put in place systems, procedures and controls that will ensure that the process continues to perform at its improved level.

Often in Poka Yoke exercises, particularly when source detection or inspection processes have been implemented, this is all the control that is needed. However the team should think about the risks of your error proof failing. Again an FMEA may help here.

When these have been identified a control plan can be developed in order to ensure that the improvements are locked in. Such controls could be; new standard operating procedures including any training and auditing to ensure that these are effective, and any checks or monitoring procedures established.

Error proofing exercises such as has been described above are surprisingly effective. As Mr Shingo once said "It is not the impact of a single Poka Yoke but the accumulation of many such devices and procedures that is almost frightening to behold".

Poka Yoke or error proving is an improvement over the traditional inspection model as it aims to stop errors being produced in the first place or eliminates them through self checking or by successive checking in subsequence steps of the process. Undertaking an error proofing exercise can be very effective if a systematic problem solving process such as DEMAIC or PDCA is used. It may need many such error proof mechanisms, the accumulated effect of which can be remarkable.

If you want to know more about error proofing your processes contact SigmaPro to find out about our comprehensive training and support programmes.

 

Author Name - David Cowburn (MBB, Lean Six Sigma)

David has 25 years of running companies to Managing Director level and is experienced in utilising Lean Six Sigma in a wide variety of businesses including, manufacturing, process industry, service, and administrative.

In a people based hands-on style, he works and trains at all levels in an organisation from Board to shop floor to bring about rapid measurable step changes in performance.

David was originally trained in the Toyota Production System and has since developed a high level blend of Lean and Six Sigma philosophies and tools through working with businesses all around the World.