← Agile and Scrum: Flexible Project Management
Lesson
Lesson 6.2 — Flow metrics, Little's Law, burndown and CFD
Read the main flow metrics and apply Little's Law arithmetic, and read a burndown chart and a CFD.
Flow metrics and Little's Law
Flow metrics and Little's Law
To manage and improve flow, teams track three core metrics. Cycle Time is the elapsed time from when an item starts (work begins) to when it finishes (delivered/done). Throughput is the number of items a team completes per unit of time (e.g., 5 items per week). Work In Progress (WIP) is the count of items that have been started but not yet finished at any given moment. A fourth metric, Work Item Age, measures how long a specific in-flight item has been in progress so far — useful for spotting stalled items.
These three metrics are connected by Little's Law, a mathematical result from queuing theory: average WIP = throughput × average cycle time. Two worked examples with whole numbers:
• Throughput = 5 items/week, Cycle Time = 2 weeks → WIP = 5 × 2 = 10 items.
• WIP = 12 items, Throughput = 4 items/week → Cycle Time = 12 ÷ 4 = 3 weeks.
The practical implication: if you want to reduce cycle time without changing throughput, you must reduce WIP. This is the quantitative foundation behind Kanban's WIP limits.
Teams also use two visual tools to track flow over time. A Burndown Chart plots remaining work (story points or tasks) on the vertical axis against time on the horizontal axis; an ideal diagonal line descends to zero at Sprint end, and deviations reveal scope changes or delivery risk. A Cumulative Flow Diagram (CFD) stacks the count of items in each workflow stage over time; the horizontal distance between the "started" and "done" bands shows average cycle time, the vertical distance shows WIP size, and a widening band in any stage reveals a bottleneck accumulating there.
Lesson notes
Flow metrics and Little's Law
To manage and improve flow, teams track three core metrics. Cycle Time is the elapsed time from when an item starts (work begins) to when it finishes (delivered/done). Throughput is the number of items a team completes per unit of time (e.g., 5 items per week). Work In Progress (WIP) is the count of items that have been started but not yet finished at any given moment. A fourth metric, Work Item Age, measures how long a specific in-flight item has been in progress so far — useful for spotting stalled items.
These three metrics are connected by Little's Law, a mathematical result from queuing theory: average WIP = throughput × average cycle time. Two worked examples with whole numbers:
• Throughput = 5 items/week, Cycle Time = 2 weeks → WIP = 5 × 2 = 10 items.
• WIP = 12 items, Throughput = 4 items/week → Cycle Time = 12 ÷ 4 = 3 weeks.
The practical implication: if you want to reduce cycle time without changing throughput, you must reduce WIP. This is the quantitative foundation behind Kanban's WIP limits.
Teams also use two visual tools to track flow over time. A Burndown Chart plots remaining work (story points or tasks) on the vertical axis against time on the horizontal axis; an ideal diagonal line descends to zero at Sprint end, and deviations reveal scope changes or delivery risk. A Cumulative Flow Diagram (CFD) stacks the count of items in each workflow stage over time; the horizontal distance between the "started" and "done" bands shows average cycle time, the vertical distance shows WIP size, and a widening band in any stage reveals a bottleneck accumulating there.