Card limits, also known as WIP (Work in Progress) limits, are a powerful way to ensure work moves smoothly through your Favro flow boards. If a column (process state) has too many cards, the people on your team responsible for finishing the work at that stage will be overwhelmed, causing a bottleneck. Their focus will change from finishing cards to starting cards and wasting time by task switching between them. The idea of any flow board is to finish the most important cards first before pulling in the next most important cards from the previous stage as capacity allows. The card limits act as a throttle at each step, which can be fine-tuned over time to optimize your throughput of cards from start to finish. Getting started with card limits is not as tricky as you might think and, as you progress, Favro makes it simple to visualize flow and refine card limits accordingly.
An excellent agile rule of thumb for setting initial card limits at each stage is to take the number of team members responsible for that stage and subtract one. Doing so will ensure that there’s slack in the flow. Team members at each step will have time for quality checks, assisting other team members and striving for continuous improvement. To set card limits click on the down arrow of each board column (your last stage doesn’t need one) and select “Set card limit…”. If you want to set them all at once, then click the board’s menu and select “Layout -> Card limits…”. Here you can also choose whether or not card limits are enforced for each column.
Over time, Favro will be automatically gathering your secure flow-based data. This data can be viewed in either a cumulative flow diagram or a control chart. The cumulative flow diagram enables you to visualize your current process flow efficiency. Are too many things coming in, without enough going out? Are you starting a lot of things and not finishing many of them?
The cumulative flow diagram built into your Favro board will answer these questions. On the board menu select “Charts -> Cumulative flow charts”. Then, all you need to do is look at the steepness of the line for the input or first column vs. the steepness of the line for the output or last column. By making this comparison, you’re comparing your arrival rate — how fast cards are added to your board — to your departure rate — how fast cards are completed. If the arrival rate is steeper than your departure rate, then you’re overburdened, and card limits need to be decreased. If your departure rate is steeper than your arrival rate, then you’re underutilized as a team and card limits should be increased. This same analysis can be done for any stage by comparing the column just before and the column just after any particular column.
Of course, the goal is for the arrival rate and departure rate to be equal. Therefore, the ideal way to fine tune your card limits at each stage is to set them to the departure rate for each stage, ensuring that work isn’t coming in faster than it can be completed.
Calculating the departure rate requires some simple high school geometry. Your departure rate is the slope of the average line, drawing a straight line between any two points on the output curve. Slope is equal to change in “y” (rise) divided by change in “x” (run).
Lets take this example, where your arrival rate is greater than your departure rate:
If your average departure line over 5 days has risen 30 cards, meaning 30 cards have been completed over the course of 5 days, then your calculation would look like this:
In this example, setting your card limit to 6 will bring the arrival rate inline with your departure rate. This will optimize your flow by ensuring that team members are not working on too many cards at once, allowing them to focus on finishing the highest priority cards at the highest level of quality.