One Piece Flow & Standard WIP

I learned a lesson about how easy it is to assume people understand something that sounds simple to you. Near the end of a recent Lean study mission in Japan, one of the participants who is a Six Sigma Black Belt, a dynamic change agent, and a PhD made the comment “We didn’t see any one piece flow this week.”
While he agree that each company showed excellent examples of Lean and kaizen, he had not seen any one piece flow. This stunned several of us at the lunch table. We had been to Toyota, among others, where things flowed one at a time. We argued heatedly for a few minutes until we realized the source of the confusion.
One piece flow does not mean there is only one piece of work on the line, or that there is only one piece of work between one worker and the next. For various reasons there can be more than one piece between processes (cure times, automatic machining process, transportation distance between processes, etc.). When these pieces are placed there logically and mathematically (based on takt time and cycle time) this is what is called Standard WIP.
Our very intelligent and observant friend had seen this Standard WIP, and not knowing what it is, had thought that there was “no one piece flow”.
The strict definition is that one piece flow is that each person or process only works on one piece at a time before it is pulled downstream, one piece at a time, to the next process. If you have multiple pieces of work in front of you that you are working on, by definition this is not one piece flow.
There is a fine distinction between “fake flow” and one piece flow with Standard WIP. If you have material or work pieces between processes for no good reason (such as auto cycle machining, cure / bake / set time, or a pull-based transport) then it is fake flow. If the material is there in a set quantity that is a factor of takt time divided into cycle time or time-based work constraint, it is still one piece flow.
Ironically, you can have one piece flow through a batch process so long as it is one piece going in and coming out of the process. This is a related, though different topic.
Suffice it to say that with a simple demonstration with sugar cubes the Black Belt was convinced that he had been seeing one piece flow all week.