The graphic on the right shows a number of diagrams all of which were derived from very simple data about each item that flowed through this system:
- when it arrived into the system;
- when it departed the system; and
- whether the item was "delivered" or "discarded".
Note: I use the term "discard" here as a general term to include an exit from the system at any point in the system and for any reason. It includes aborting/abandoning the item after commitment, as well as postponing the item by moving it back to a part of the process upstream from the system under study. For the definition of this and other terms used here please see this Glossary.
- Average Delivery Rate. This is measured in items per week, and the average is taken over 1 week. Note this only shows actually delivered items. Alternatively a plot of "Throughput" could have been used which includes all items that have passed through the system.
- Average Time in Process (TiP). This is measured in weeks and again the average is taken over 1 week.
- Average Work in Progress (WiP). This is measured in number of items, again averaged over one week. Care must be taken when calculating average WiP for a day, particularly on days when an item arrives in or departs from the system, to ensure that it is consistent with the calculations of average TiP.
- Net Flow. Simply the difference between the number arriving and departing over the previous week.
- Delivery Bias. This is a measure of the degree to which Delivery Rate is higher or lower than would be predicted by Little's Law for the given period (1 week in this case). If it is non-zero it indicates away from stability. Further discussion of this quantity is found here.
- Flow Debt/Credit. This is a measure of the degree to which the average TiP varies from that predicted by the CFD. This also indicates a degree of instability if it varies significantly from zero. See Dan Vacanti's book [vaca] for further discussion.
- Age of WiP Indicator. This compares the average age of the WiP with half the average Tip. It is another indicator of imbalance.
- [vaca] Vacanti, Daniel S. "Actionable Agile Metrics for Predictability: An Introduction". LeanPub. (2015)