Cumulative-Flow Diagram

A cumulative-flow diagram (CFD) demonstrates the arrivals to and departures from a system over time, as well as Throughput and Delivery Time, and as such as one of the best tools for visualizing flow.

A CFD brings together the Big Three Flow Metrics into one handy chart:

  • Throughput: The slope of the lower line (i.e., items per day or week, etc.)
  • Work in Progress: The vertical reading between the “done” (departures) work line and the top (arrivals) line. In this example, on 2/23, the system’s work in progress amount is 24 items.
  • Delivery Time: The horizontal reading between the “done” (departures) work line and the top (arrivals) line. In this example, on 2/23, the average approximate delivery time was eight days.

Little’s Law

A cumulative-flow diagram displays Little’s Law in one picture.

Little’s Law Assumptions

  1. The average input or arrival rate of a process should equal the average output or departure rate.
  2. All work that is started will eventually be completed and exit (depart) the system.
  3. For the time period under consideration, the average age of WIP should neither be increasing nor decreasing.

Properties of CFDs

From Daniel Vacanti’s When Will It Be Done? book:

  1. The top line of a CFD always represents the cumulative arrivals to a process, and the bottom line always represents the departures.
  2. Due to its cumulative nature, no line on a CFD can ever decrease/go down.
  3. The vertical distance between any two lines is the total amount of work that is in progress between the two workflow steps represented by the two lines
  4. The horizontal distance between any two lines represented the average approximate delivery time for items that finished between the two workflow steps represented by the two lines
  5. The data displayed depicts only what has happened for a given process. Any chart that shows any type of projection is not a CFD.
  6. The slope of any line between any two reporting intervals represents the exact average arrival rate of the stage represented by the succeeding band.

Sources and Resources