Wednesday, April 23, 2008

Detecting Bottlenecks in a Kanban System

How do you identify a bottleneck in a kanban system? For the last 5 years, I’ve been teaching teams to identify bottlenecks using Cumulative Flow Diagrams (CFDs), Figure 1. A growing gap shown as a bulge in a colored section of the graph indicates growth in work-in-progress (WIP). And this illustrates where the bottleneck is to be found.

Figure 1. Cumulative Flow Diagram showing “design” step as a bottleneck

However, in a kanban system the WIP is limited and the CFD (Figure 2) does not typically exhibit a bulge like the one in figure 1.

Figure 2. Kanban system CFD with no noticable bottleneck indicator

So how do you detect bottlenecks in a kanban system?

Bottlenecks are not indicated by inventory build up. They are indicated by ragged flow of WIP through states in the process. When things are not moving smoothly a bottleneck is indicated. There are two ways to see this. One is dynamic, by looking at the kanban board every day at the standup meeting and observing that things are not moving. The second is to observe vacant space on the kanban board - WIP dropping to zero at a given state or set of states in the process, as seen in Figure 3.

Figure 3. Kanban board showing a section of zero inventory indicating a bottleneck in front of this area

As work continues to be pulled through to the end of the board - the software release - a bottleneck will be indicated by an opening gap in WIP. Things cannot be pulled through the system despite downstream capacity because of the bottleneck.

Hence, in a kanban system the CFD behavior is inverted. A band of zero inventory on a CFD would indicate a bottleneck directly before it. (Unfortunately, I don’t have a good report graphic to demonstrate this. I’ll need to make one.) Technorati tag: David+Anderson, Software+Engineering, Kanban, Lean