Jun 21 2014
Poka-Yoke in User Interface Design | Six Revisions
“Poka-yoke is a Japanese term that means “mistake-proofing”. It surfaced in the 1960s, and was first applied in the car manufacturing industry. Poka-yoke is credited to industrial engineer Shigeo Shingo.”
Source: sixrevisions.com
That IT specialists should be interested in the Poka-Yoke concept is natural. There are, however, consequential inaccuracies In the way it is described in many English-language sources, including Wikipedia.
The example given as the first Poka-Yoke is a redesign of a switch assembly process that involved presenting springs on a placeholders so that the operator would not forget to insert one.
Assuming this is a true example, it has two characteristics that make it different from the other examples given in Shingo’s book or in Productivity Press’s big red book of Poka-Yoke.
First, having a placeholder does not physically prevent the operator from making a mistake. A classical example of a system that does is one that puts a lid on every bin except the one the operator needs to pick from.
Second, this example adds labor to the operation, which means that the preparation step of placing the springs in the placeholder is likely to be by-passed under pressure. This is why it is a requirement for a Poka-Yoke not to add labor to the process.
For the same reasons, a multi-step deletion process in a software interface does not qualify as a Poka-Yoke. If you do multiple deletes, you end up pressing the buttons in rapid succession, occasionally deleting items you didn’t intend to, while cursing the inconvenience of these multiple steps.
Having different, incompatible plugs certainly made it impossible to plug the keyboard into a port for an external disk. USB, however, was an improvement over this, because, with it, the machine figures out the purpose of the connection. A connector that you can insert in any orientation is even better. It saves you time, and there is no wrong way to plug it in. This is a genuine Poka-Yoke.
There are other, useful approaches that make mistakes less likely without preventing them outright. Don Norman and Jacob Nielsen call them “usability engineering.” They should certainly be used in user interface design, but not confused with Poka-Yoke.
June 21, 2014 @ 12:32 pm
Achieving Zero Defects of actions and items..
I had an opportunity in 1989 to discuss Poka-Yoke with Shingo himself. Below are extracts from my notes on his comments, and my own experiences over the last 25 years. Initially he chided me for talking about it as an individual technique, and said it should be seen as the tool for implementing his system of ‘source inspection’ and guaranteeing zero defects. —
Shingo explained that traditional ‘long cycle’ inspection systems wait until an error in action produces a defective item, the defective item is then found by inspecting the output. His concept of source inspection uses the ‘short cycle’ inspection system. In this system the action itself is checked 100% using mechanical means. If an error occurs, immediate action is taken to correct it before a defect is produced. With this methodology we can guarantee zero defects to the final customer. —
The basic system is simple;
The Poka-Yoke methods/devices should be designed to detect deviation from the standard actions and outputs required to satisfy the customer’s requirements. *
This can be done in three ways;
a) Physical contact.
b) Fixed values.
c) Motion steps.
In some cases at the original design stage the part can be made a Poka-Yoke device by ensuring it can only be assembled/used in the correct way.
They should also check for deviation in the 3M’s of actions and items;
Missing. Action or item not there.
Misplaced Action or item there, but in wrong position.
Malformed. Action or item are there but wrong, size, shape, colour, temp etc.
When designing Poka-Yoke devices they must check for specific deviations in the 3M’s using; a, b and c. This can be done with a ‘what can’ 3 M’s analysis.
The Poka-Yoke device should then;
1) Control the operation. Stop the process when an error or defect occurs.
2) Warn the operator. Signal to the operator that an error or defect has occurred.
They should be applied at the following check points;
1) The source action. (source check) This is the ideal as it gives zero defects.
2) Output of the action. (self check) . This is our second choice as the output will be defective if the PY device is activated, but it will not be passed to the internal customer.
3) Before the next process. (successive check). At this stage the item will be defective if the PY device is activated, but it cannot go to the final customer.
With this system in place it is now possible to consistently achieve;
‘Zero Defects in our activities and production processes’.
This was Shingo’s original goal in 1965.
If applied to safety it is possible to achieve ‘Zero Accidents’. I do not understand why this methodology is not more widely used in this area.—
The most impressive example of Shingo’s system I have experienced was on an assembly line for inlet manifolds in Japan. We were allowed to work on the line and challenged to produce a defective assembly. It was impossible to produce one, and we had some very talented people trying. —
Poka-yoke should be seen as the device for implementing Shingo’s zero defects system.
The goal is to identify deviation from the desired conditions or actions in any situation.
A good example is the selector stick on an automatic car gearbox. If the stick is not in the park position the poke-yoke switch is not activated and the engine will not start. Zero defects in all situations. Shingoe pointed out to me that this would be impossible to achieve with statistical techniques.