Which parameter is appropriate for CFD?
I was looking into Cumulative Flow Diagrams (CFD) for Scrum teams and I noticed that there are multiple ways the CFDs can be plotted. I understand that the X-axis should be the time scale to understand the workflow but I have some confusion regarding the other parameter on the Y-axis.
In this blog the parameter used is Work Item where as in some other resources the parameter varies and sometimes "estimated work" is used as the parameter. As per my understanding the estimates are just the forecast and may not truly represent the value (unless the work completed is precisely as estimated).
Is it better to use work items (or PBIs) or estimates?
My confusion is because it is a flow diagram and continuous, so re-estination would be hard to factor in. Similarly, just using the work items may not represent actual size of the work item.
Which of these would allow you to make the most meaningful calculations? Consider average cycle times, approximate average cycle times, and the calculation of flow debt for example.
Why use estimates when they are guesses? Every Cummulative Flow Diagram I have ever used had actual number of work items represented on the Y axis and "work items" are represented by the items that are actually delivering value. So if your team uses User Stories and then subtasks, the subtasks are not included in the count because the User Story is the item that actually represents the value delivered to the stakeholder/end users. If you create bugs they would be represented in the item counts but only those that are found in Production and not the ones found during the development of associated to a User Story. The second set I mentioned would be considered part of the work done to deliver the User Story's value.
By representing the actual count of items you are showing your real flow of delivered value. If you use estimates you are only showing your flow of relative guessing.
Aditya - What are you hoping to accomplish with the CFD? It's and "okay" chart to show WIP. If you are using Scrum with Kanban, I would suggest there are even better alternatives, like scatter plots, throughput charts, and work item aging.
Some additional resources: https://www.scrum.org/resources/suggested-reading-professional-scrum-kanban