Weighted Shortest Job First

Definition

When jobs differ in both duration and cost of delay, Reinertsen shows the economically optimal sequencing rule is to prioritize by cost of delay divided by duration — weighted shortest job first (WSJF). This generalizes two simpler special cases: when delay costs are equal across jobs, do the shortest job first (SJF) to avoid the "convoy effect" where one slow job delays a queue of fast ones behind it; when durations are equal but delay costs differ, do the highest cost-of-delay job first (HDCF).

In the Book

Chapter 7 builds WSJF through three principles (F15–F17), each illustrated with a cumulative delay-cost diagram: processing jobs in a 1–2–3 order (shortest first) produces a smaller total shaded "delay cost" area than a 3–2–1 order, because delaying many short jobs behind one long one compounds their collective wait. The chapter names this the convoy effect, comparing it to one slow car on a narrow road delaying every car behind it. F17 combines both factors into the single WSJF priority score and calls it "the most complex situation and the most common one" in real product development, where jobs vary in both dimensions simultaneously. Reinertsen then contrasts WSJF against the sequencing rules companies actually use: prioritizing by ROI alone (which ignores how sequence changes delay costs — the book notes overall portfolio ROI adjusted for delay cost matters more than any single project's ROI), plain FIFO (dangerous once delay costs differ, likened to an emergency room that ignores triage), and Critical Chain.

Why It Matters

WSJF converts "how should we sequence this backlog" from a debate about competing priorities into an arithmetic sort — divide cost of delay by duration, rank descending — that any team with the two inputs can run without escalating to management. It later became the standard prioritization formula in the SAFe agile framework, but the underlying insight generalizes past software backlogs to any queue where items differ in both how expensive delay is and how long they take to clear: triage, scheduling, and portfolio sequencing all reduce to the same rule once you accept that both variables matter and interact.