The 4 terms are determined by Planning Poker estimation using a Fibonacci scale (usually 1 to 20). The work items are ordered according to the formula following an estimation workshop. While this formula cannot provide a true quantitative analysis for ordering items to maximise value, some consultants using it have said that is a useful technique for the discussion it engenders among stakeholders. Once the numbers have been generated the items ordered, re-ordering to a better order is straightforward because of all the discussion that has preceded this point. Needless to say, I strongly disagree with this. While the discussion is necessary for a quantitative or qualitative approach, creating a spurious anchor from numbers which cannot be meaningful will lead to cognitive bias rather than better ordering.
Why can't the above formula provide meaningful answers? For 2 reasons:
- Dimensionally the formula is inconsistent
- The terms are not estimated on a proportional scale
"WSJF" = Time Criticality x (User-Business Value + Risk Reduction | Opportunity Enablement) / Size
- Time Criticality, τ, has units of the reciprocal of time (e.g. days-1). In other words an option expiring in 2 months would have double the τ of one expiring in 4 months.
- User-Business Value and Risk Reduction | Opportunity Enablement are measured in consistent units, possibly using a weighting factor to translate the intangible values to the units of the tangible values
- Size is proportional to the blocking time caused by implementing the item. This term may in such a case be used as a proxy for duration measured in units of time. This issue has been addressed in this series here: WSJF - Should you divide by Lead Time or Size? which also identifies additional assumptions required for this to be true.
- WSJF itself is used consistently with its intended dimensions, which are value/time2
The modified SAFe formula suggests a more general expression for WSJF using the weighting factors for a set of "business value types", v, and "exchange rates" that convert the values to a common "currency" of value:
WSJF = τΣ(vn Xn) / D
τ (tau) is time criticality, vn is the nth business value, Xn is the exchange rate for this business value type, D is duration, for which Size may be a proxy subject to the assumptions discussed above.
This blog has not addressed the issue of when quantitative or qualitative approaches should be used. As well as having formulae that are coherent, the work of estimating to provide numbers for the formulae must be worthwhile and comprehensible to the business doing such estimation. In many cases it is not - for example where the domain or context in inherently "non-plannable". The concept of cost of delay is still important, but we should for different techniques for ordering work. Further discussion of this must wait for the next article in the series.
Part 1: Understanding Cost of Delay and its Use in Kanban
Part 2: Delay Cost and Urgency Profiles
Part 3: How to Calculate WSJF
Part 4: WSJF - Should you divide by Lead Time or Size? (this article)
Part 5: A "Qualitative" Formula for WSJF? (this article)
Part 6: Time is an asset: delay is a cost (to be published)
 David J. Anderson and Andy Carmichael, Essential Kanban Condensed. (United States: Lean Kanban University Press. 2016)
 Anderson, David. 2015. ESP: Scaling the benefits of Kanban. Slides 45-49. April 23. http://www.slideshare.net/agilemanager/enterprise-services-planning-scaling-the-benefits-of-kanban (January 5, 2017).
 Sharvari, Sawant. 2016. "SwiftKanban help - risk module.". http://help.swiftkanban.com/getting-started/projects/metrics/esp-analytics/risk-management.html(January 5, 2017).
 Magennis, Troy. 2016. Better Backlog Prioritization (from random to lifetime cost of delay). https://github.com/FocusedObjective/FocusedObjective.Resources/raw/master/Canvas%20and%20Forms/Better%20Backlog%20Prioritization.pdf
 Agile, Scaled. 2016. "WSJF – scaled agile framework.". http://www.scaledagileframework.com/ wsjf/ (January 5, 2017).