Queues as the Root Cause

Definition

Queues — work waiting for a busy resource — are, in Reinertsen's framing, the primary source of economic waste in product development, more damaging than in manufacturing because they are both physically and financially invisible. Unlike a factory's work-in-process inventory, which a CFO can read off the balance sheet, "design-in-process" inventory shows up nowhere in the accounts, so it accumulates unmanaged, unmeasured, and often unnoticed.

In the Book

Chapter 3 borrows vocabulary from queueing theory (Erlang's 1909 work on telephone networks; the M/M/1/∞ queue) to argue that product development's real problem is periods of inactivity, not slow activity. Principle Q1, "The Principle of Invisible Inventory," uses an anecdote from a Hewlett-Packard engineering manager — "our inventory is bits on a disk drive, and we have very big disk drives" — to show why queues escape scrutiny: they leave no physical trace the way a factory floor's stacked inventory does. Principle Q2 traces six distinct kinds of damage that queues cause: they increase cycle time (a quantifiable cost-of-delay effect), raise risk by lengthening exposure to shifting requirements and competitor moves, amplify variability at high utilization, raise reporting overhead (the book contrasts a 16-week-lead-time factory needing 60 status transactions against a lean factory needing two), and — most importantly for quality — delay the feedback that catches bad assumptions before they compound.

Why It Matters

Most organizational diagnosis stops at symptoms (missed deadlines, low quality, high overhead) and proposes symptom-level fixes (more status meetings, more testing, more resources). Naming queues as the shared root cause redirects attention to the one lever that, if pulled, improves cycle time, risk, cost, and quality simultaneously — and it explains why adding resources to a bottleneck without shrinking the queue in front of it often fails to help. It also generalizes past software and hardware development to any system where "inventory" is information rather than physical stock, and therefore easy to accumulate without anyone noticing.