Estimation techniques

Estimation techniques

Many decisions need a number that nobody knows exactly: how much traffic, how long the project, how big the market. Good estimation gets within an order of magnitude fast by decomposing the unknown into knowable parts, anchoring on real reference data, and stating the uncertainty honestly rather than pretending precision.

Method

1. Decompose with Fermi estimation. Break the unknown into a product of factors you can each estimate to within a factor of a few: "requests per day = users x sessions per user x requests per session". Errors in independent factors partially cancel, so a product of rough estimates is often within 2-3x of truth (the classic "piano tuners in Chicago" method: see the decomposition instinct in first-principles-thinking, capacity-planning's load model).
2. Anchor on reference classes, not intuition. Instead of estimating this project's duration from scratch, ask how long similar projects actually took (reference-class forecasting): the outside view beats the inside view, which is systematically optimistic (the planning fallacy). "Projects like this took 3-6 months" is more reliable than "this one feels like 6 weeks" (see the optimism correction in status-updates).
3. Bound with best/worst/likely. Estimate a range, not a point: the plausible low, high, and most-likely: because a single number implies a precision you do not have and invites false confidence. The range is the honest answer, and its width tells the decision-maker how much the estimate can be trusted (see tradeoff-analysis's sensitivity).
4. State error bars and carry them. An estimate without uncertainty is a lie of precision; "roughly 10,000, could be 5,000-20,000" is more useful than "10,000" because it tells you whether the decision is robust to the uncertainty. Propagate the uncertainty through the decision (if the answer flips within the error bars, you need a better estimate: see experiment-design-basics).
5. Sanity-check against a second method and known bounds. Estimate the same quantity a different way and see if they agree (order-of-magnitude agreement is reassuring; wild disagreement means a factor is wrong); check against hard limits (an estimate exceeding the total market or the physical maximum is wrong somewhere). Cross-checking catches the decimal-place and unit errors that single estimates hide.
6. Calibrate over time. Track your estimates against actuals (see decision-journals): are your ranges right about as often as their confidence claims, or are you overconfident (ranges too narrow: the common failure)? Calibration is trainable, and estimators who track their record get measurably better at sizing uncertainty.

Boundaries

• Estimation gets you a defensible number for a decision, not a precise fact; treat estimates as inputs with uncertainty, not truths, and revisit them as real data arrives (see capacity-planning's re-forecasting).
• Some quantities have fat tails or fundamental unpredictability (novel systems, black-swan events) where point estimates mislead badly; there, plan for ranges and robustness rather than betting on the estimate (see the degradation planning in scalability-planning).
• Anchoring is a double-edged tool: a bad reference class or a planted number biases the estimate. Choose reference classes deliberately and beware anchors others supply (see the negotiation anchoring in salary-negotiation).