Estimate conservatively and act on the conservative number

When uncertain, use a pessimistic estimate as your working assumption — not your best guess.

Why it works

The planning fallacy (Kahneman & Tversky) shows that people systematically underestimate time, cost, and difficulty. Using a conservative estimate corrects this systematic bias: the downside error is missing an upside that did not materialize; the upside error from an optimistic estimate is a shortfall you have to absorb. When errors are asymmetric in consequence, conservative estimates are the rational choice.

How to do it

1. Make your best estimate, then ask: what would it be if I assume things go 30% worse than expected?
2. Use that pessimistic number as your planning baseline.
3. Treat any performance better than your conservative estimate as a positive surprise, not a confirmation of optimism.
4. Reserve optimistic projections for aspirational goals, not operational plans.

Evidence

The planning fallacy — systematic underestimation of time and cost — is one of the most replicated findings in judgment and decision-making research. (observational)

The planning fallacy is robust for projects with many interdependent steps; for simple, familiar tasks where feedback is rich, overestimation can occur instead.

Sources

• Kahneman & Tversky (1979), intuitive prediction and biases in planning, Psychology of Prediction
• Buehler, Griffin & Ross (1994), "Inside the Planning Fallacy", Journal of Personality and Social Psychology

Common mistake

Making a conservative estimate and then quietly using the optimistic number when making commitments, treating the conservative estimate as a hedge you have already fulfilled.