Why Use Story Points Instead of Time and How to Estimate Them with the Fibonacci Sequence

1. Why use story points instead of time

Story points provide an abstract unit of work that allows developers of varying skill levels to reach a quick consensus on effort, similar to using kilometers to describe distance regardless of speed.

2. Why use the Fibonacci sequence for story‑point estimation

Human perception follows Weber's law: differences below a certain percentage are indistinguishable. The Fibonacci numbers increase by roughly 60 % after the first few values, matching the golden‑ratio threshold where most people can reliably tell the difference.

3. Why a modified Fibonacci sequence is used

Teams replace the later Fibonacci values (21, 34, 55…) with 20, 40, 100, etc., because larger numbers become increasingly imprecise; the bigger jumps (100 % or 150 %) are easier to differentiate and still respect Weber's law.

4. Why planning poker is employed

Planning poker mitigates the “herding effect” by having each participant reveal their estimate simultaneously, often using anonymous electronic cards, thus preventing premature convergence on a single value.

5. How to estimate story points

Estimation starts with a baseline story worth one point (e.g., a login feature). Other stories are compared to this baseline based on three factors: amount of work, risk/uncertainty, and complexity. Relative sizing, discussion of outliers, and averaging followed by rounding to the nearest Fibonacci value produce the final estimate.

6. Practical tips and FAQs

Estimation should occur in dedicated sessions (not during sprint planning), only the development team participates, and large stories should be split. Adjustments after a sprint are discouraged because story points are a planning aid, not a performance metric.