My own particular interest in flow systems is the management of agile software development teams, usually using some variant of Scrum and/or Kanban, and other agile practices such as test-driven (or automated-test intensive) build-test-deploy processes. However the discussion is relevant in many other domains, such as one I've recently been involved in discussing, the flow of patients through diagnosis, treatment and convalescence in healthcare systems.
In managing these systems we need ways to look at the mass of data that emerges from them to focus on the useful information rather than the noise; information in particular that indicates when intervention is appropriate to improve flow, and when the attempt would be as futile as trying to smooth the waves on an ocean. Flow systems in knowledge work contain variability. That variability, within certain bounds (much wider bounds than in manufacturing for example), is desirable to allow innovation, responsiveness and minimising wasteful planning activities.
In this context Flow Debt is a measure that provides a view of what is happening inside our system. This is in contrast with other important measures such as Throughput (Th) and the time an item stays in the process (I call this "Time in Process", TiP [macc], though other terms may be used). These measures provide information only after items have left the system, which may be too late to avoid problems accumulating.
Having Flow Debt roughly translates as: delivering more quickly now at the cost of slower times later. It is calculated by comparing the time since the number of arrivals into the systems was equal to the current number of deliveries with the average time in the process for the most recent deliveries. It is easiest to visualise this on a Cumulative Flow Diagram.
Flow Debt = (Time since number of arrivals equalled deliveries) - Average TiP
- [macc] Maccherone, Larry. Introducing the Time In State InSITe Chart. LSSC. (2012)
- [vaca] Vacanti, Daniel S. Actionable Agile Metrics for Predictability: An Introduction, LeanPub. (2015)