WSJF model for prioritizing project and product tasks. Weighted Shortest Job First (WSJF) is a prioritization model used to sequence jobs (eg features, capabilities, and epics) to generate maximum economic benefit. In the SAFe framework, the WSJF model is estimated by dividing the cost of delay (CoD) by the task size.
In a flow-based system, updating priorities continuously provides the best economic results. In such a context, it is the sequence of work that produces the best result, rather than the theoretical return on investment.
For this purpose, the SAFe framework uses WSJF to prioritize backlogs by calculating the relative cost of delay (CoD) and job size. Backlog priorities are continuously updated based on relative user and business value, time factors, risk mitigation and enablement opportunities, and relative task size. The WSJF model also conveniently and automatically ignores sunk costs, a basic tenet of lean economics.
The details – WSJF model for prioritizing project and product tasks
Reinertsen describes a comprehensive model called WSJF for prioritizing tasks based on the economics of lean product development flow. The WSJF model is calculated by dividing the cost of delay (CoD) by the duration. Delay cost is the money lost by delaying or not doing something after a certain period of time. For example, if an upcoming feature is worth $100,000 per month and is three months late, the cost of the delay will be $300,000.
The tasks that can provide the most value (or cost of delay) in the shortest time have the best economic returns. As applied to the SAFe framework, this model supports some additional product development flow principles, including:
- Considering an economic perspective
- Ignoring sunk costs
- Financial choices on an ongoing basis
- Using decision rules to decentralize decision-making and control
- If you only quantify one thing, it should be the cost of delay
The image below shows the effect of correctly applying the WSJF Reinertsen model. The areas shaded in blue represent the total delay cost in each case. Doing the shortest weighted task gives the best economic case by a very large factor.
(Note: As shown in the figure below, Reinertsen uses actual monetary values for delay cost and estimated task duration, while the SAFe framework applies relative estimation using a modified Fibonacci sequence, which is explained later in this article.)
Using the WSJF algorithm provides the best overall economy
Estimating the cost of delay
In the SAFe framework, “tasks” are features, capabilities, and epics that live in their respective backlogs. However, since it can be challenging to determine the total delay cost for tasks that have never been executed, SAFe uses a proxy for delay cost that estimates the size of the task relative to other tasks on the list. Three main components contribute to the cost of delay:
- Business-User Value: What is the relative value to the customer or business? Do our users prefer this to that? What is the impact of income on our business? Are there potential penalties or other negative effects if late?
- The importance of time: How does user-business value degrade over time? Is there a specific deadline? Will they wait for us or will they turn to another solution? Are there critical path milestones affected by this issue? What is the impact on customer satisfaction now?
- Mitigating Risk-Value Enabling Opportunity: How else does this impact our business? Does it reduce the risk of this delivery or future deliveries? Is the information we will receive valuable? Does this feature provide new business opportunities?
Teams compare backlog items against each other using the same modified Fibonacci numbers as the Poker Guess. Then the delay cost (relative) is calculated as follows:
Estimating the duration of work
The next item in the equation is the duration of the job. It can also be very difficult to determine, especially early on, when it is hard to tell who will do the work or what capacity allocations can be made. Fortunately, task size is a good proxy for the duration. (If I’m the only one mowing, and the front yard is three times bigger than the backyard, the front takes three times as long.) Using job size, we have a simple calculation to compare jobs via the WSJF model.
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