Three-point estimation: (optimistic + 4x realistic + pessimistic) / 6 — counteracts the optimism bias inherent in single-point estimates

Why This Is a Rule

Single-point estimates ("This will take 3 hours") anchor on the best-case scenario because that's the scenario your brain simulates most easily. You imagine the task going smoothly: no interruptions, no unexpected complications, no rework. This produces a systematic optimistic bias — the "planning fallacy" — that underestimates duration by 30-100% for most tasks.

The PERT (Program Evaluation and Review Technique) three-point estimation forces you to explicitly consider the range: Optimistic (everything goes perfectly — best realistic case, not fantasy), Realistic (most likely scenario including normal friction), and Pessimistic (significant complications — worst realistic case, not catastrophe). The weighted formula (O + 4R + P) / 6 produces a beta distribution that accounts for the asymmetry of duration distributions: tasks can take much longer than expected but rarely finish much faster than the optimistic case.

The formula weights the realistic estimate 4x because most outcomes cluster around the most-likely scenario, but the inclusion of optimistic and pessimistic estimates widens the final number in the direction of actual probability. The result is consistently more accurate than a single-point estimate because it forces the estimator to confront the range of possible outcomes rather than anchoring on best-case.

When This Fires

• When estimating any significant task (2+ hours of expected duration)
• When a single-point estimate feels suspiciously precise or optimistic
• When estimating for commitments where accuracy matters (deadlines, client promises)
• Complements Record estimate before and actual time after every 30+ minute task — two weeks minimum builds your personal estimation ratio (estimate tracking) with the structured estimation method

Common Failure Mode

Optimistic and pessimistic estimates that cluster too close to the realistic: "Optimistic: 2.5 hours. Realistic: 3 hours. Pessimistic: 3.5 hours." This narrow range indicates you're not actually considering different scenarios — you're anchoring on 3 hours and adding/subtracting a token amount. The pessimistic estimate should reflect genuine complications (rework, blocked dependencies, misunderstood requirements), which typically double or triple the realistic estimate.

The Protocol

(1) For each significant task, generate three estimates: O (optimistic — everything goes right, you're focused, no complications), R (realistic — normal interruptions, typical friction, standard rework), P (pessimistic — significant complications, blocked dependencies, major rework). (2) Calculate the weighted average: (O + 4R + P) / 6. This is your planning estimate. (3) Sanity check: is P at least 2x O? If not, you're probably not being genuinely pessimistic. Think of what could actually go wrong. (4) Use the PERT estimate as your commitment number, not the optimistic or even realistic estimate. (5) Track PERT estimates against actuals (Record estimate before and actual time after every 30+ minute task — two weeks minimum builds your personal estimation ratio) to calibrate: if PERT estimates are still systematically low, your "realistic" scenario is actually your optimistic scenario — recalibrate.