The teaching of lean concepts is typically tuned to continuous processes: Day in, day out, value flows continuously from suppliers until the final product reaches the customer. The concepts of lead time (the time it takes individual “flow-units” to travel through a process), Takt time (the available time divided by customer demand), and many other lean concepts are based on the assumption of continuous processing. The concept of “available time” helps expand that to interruptions in the process, such as work being done in one or two shifts only.

However, this extension is only valid if how a shift starts and how a shift ends can be neglected in the face of what is going on during the shift. Work in progress must also be virtually “frozen” between shift end and shift start. The less that is true, the more analysis tools must be specifically tuned to discontinuous processes.

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