Cumulative Flow Diagrams (CFD) Explained

This page has been created following attending this meet-up Interpreting Cumulative Flow Diagrams (CFDs) with Mark Grove

We all work in a flow fashion, and we know optimising flow leads to higher throughput.

Why is this important?

If valuable work is flowing through a system, optimising flow could lead to the delivery of value sooner. This is important because there is always a desire to finish ‘faster in’ product and software development.

A Cumulative Flow Diagram (CFD) visualises flow through a system.

From a Cumulative Flow Diagram, you can identify recurring patterns, highlight bottlenecks and their impact and also provide insight into predictability.

What is a CFD?

1. The columns/statuses that relate to work that has started are represented in the bands that you see in a CFD
2. X-axis is marked by time
3. Y-axis represents the number of items flowing through the system

A CFD has embedded within it 3 key flow metrics:

1. Work in Progress – how much WIP do you have in the system
   1. Take a look at the vertical distance of a particular band or bands representing ‘in progress’ statuses
2. Delivery Rate (cycle time – time to complete a task (based on the statuses that are deemed in progress in your system))
   1. Cycle time is from the moment something enters your system until it is done. The average cycle time can be seen in a CFD by drawing a horizontal line between the band where work starts and the done band. If you then dot down to see what the dates are in the x-axis and calculate the time between the two, this provides your average cycle time.
   2. You can also easily identify average lead time (the total time it takes for items to flow through the system from start to finish)
3. Throughput – number of items completed in a period. It’s a rate. It’s rise over run.
   1. In a CFD throughput is the slope of the done line.
   2. You can take the items from the WIP calculation
   3. And the number of days from your cycle time
   4. You then divide the WIP over the number of days and find out how many items you completed in that period.
   5. You can then divide further if you want to work out how many items for a smaller period.

How to Interpret CFD

There are a lot of patterns/prompts that you will begin to notice when working with Cumulative Flow Diagrams. A good way to think about using the data is to break the observation down into three steps:

1. What are you seeing?
   1. For example, what is the arrival rate vs departure rate?
      1. Are we starting more than we are finishing?
      2. Are we running out of work to do and the rate of completion is a lot faster than arrival?
      3. Is there a particular band (part of the process that is getting bigger and bigger)?
2. Why might that be happening?
   1. For example, if there is a particular band growing is that because
      1. There are no WIP limits?
      2. Management is pushing work onto terms
      3. Teams are not respecting the pull system or understanding how to regulate work coming in
3. This leads to what you might suggest.
   1. For example following the above example, you might suggest introducing WIP limits as a starting point.

Common Explanations/ Patterns

• If you see cut-in (bands disappearing altogether) this would be a sign to ask the question are particular bands (parts of the process) being skipped altogether?
• If you see steps like / plateau bands – stair step approach it would signal sprints/opening work and closing work.