How to read a cumulative flow diagram

Cumulative Flow diagrams can contain quite a lot of information for observations. However, they can also be a little daunting for those who are not familiar with them. In this blog post I’ll share with you some tips and information for how to start to use cumulative flow diagrams.

Here’s an example of a CFD:

Each line is a count of the number of tickets moving through that part of the process – stacked on top of one another, hence the name “cumulative”.

  • Light Blue – Done items
  • Yellow – Deploying
  • Grey – Testing
  • Orange – Developing
  • Dark Blue – Analysing

Along the x-axis are the days – it’s quite common for CFDs to use days as the point of measurement.

Along the y-axis is the count of the tickets. For example, on day 0 there were no tickets in the Done area, whereas on day 1 there were now 3.

That’s actually one key part of being able to read these kinds of graphs. The distance in height between the two lines represents the number of tickets. Thus the larger the gap between the lines, the more tickets that are in that part of the process. Therefore, the further apart the lines the greater the Work in Process (WiP).

If you get lines that are continuing to grow apart, the you have a WiP growth problem and this is going to slow down your overall throughput. This is particularly useful if you use the top and bottom lines when viewing WiP growth because these represent the input and output of your overall system. If you’re not getting growth in you system, but lines within it a growing, it means that there’s likely a bottleneck inside the system.

Additionally, where these lines start to converge, it means you have decreasing WiP and may lead to starvation of a part / all of the system. This might not be a problem in the immediate short term, but left unchecked it may lead to an unnecessary spend / costs.

Using these observations, you can start to adjust WiP limits and control the point of input to either prevent extra work coming into the system / parts of the system, or alternatively allowing more work to flow through. Effectively, this is what managing flow is about – trying to match variations in demand and capability to achieve a balance.

Another key observation about CFDs is that the average lead time is denoted by the width of the graph. For example, if you look at the above graph at the dark blue mark on day 1 – this is when item 11 entered the system. If you track horizontally across the chart, you’ll see that this item left the system (the light blue line) on day 7. To determine the system lead time, you can deduct the entry time from the exit time giving you a lead time of 6 days.

Of course, this time will vary as things change in the system. If you look further up the chart to day 7 and the analysis count there and then track across to done, you’ll notice the lead time is down to approximately 4 days. That’s about a 30% reduction in lead time and can often be attributed to actions such as limiting WiP in the system (as Little’s law tells us that decreasing WiP can also lead to decreased lead time) or other improvements to the system.

The third key attribute of this is the angle of the bottom line of the graph. This is known as the delivery rate or throughput of the Kanban system (as an average). The flatter the angle, the less the throughput. The greater the angle (moving towards vertical), the greater the delivery rate / throughput.

You can see these three key attributes in this chart:

You don’t need to always have electronic tools – if you have physical boards you can maintain a physical chart with this data. Alternatively, you can capture this in a spreadsheet. There are also other tools out there such as SwiftKanban, Kanbanize and LeanKit that do these things for you as well.

You can now start to use cumulative flow diagrams to help make more informed decisions about your system of work. You can see bottlenecks that you can action, you can start to limit or increase input to balance flow and you can limit work through the process to help smooth the flow. You can also start to make plans around how to reduce lead time or increase throughput which will have positive impact to your customers and your financial bottom line.

The cumulative flow diagram is an essential tool for managing flow and improving the outcomes of your service delivery. If you aren’t using one, you now have the basic knowledge to get started.

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