Planning Fallacy

Kahneman and Tversky's finding that people systematically underestimate the time, cost, and risk of future actions while overestimating their benefits—even when they have direct experience with similar past projects that overran. The mechanism: planners use an 'inside view' (imagining the specific steps of THIS project) rather than an 'outside view' (looking at how similar projects actually turned out). The inside view is inherently optimistic because it imagines the plan working; the outside view incorporates all the ways plans typically fail. The planning fallacy persists even when people know about it, because the inside view feels more relevant and specific.

When to use it

When project estimates need to be realistic rather than aspirational; when past projects have consistently overrun their estimates; when approving budgets and timelines that will be held to account; when designing project management processes that need to produce reliable forecasts.

How it can help

Apply the 'outside view' to every plan: before trusting your bottom-up estimate, check how long similar projects actually took. Reference class forecasting: identify a reference class of comparable past projects, determine their actual outcomes, and use that distribution as your baseline—then adjust for project-specific factors. The practical rule: your initial estimate is probably 50-100% too low. Multiply by 1.5-2x as a starting correction. Track your actual vs. estimated times over time to calibrate your personal bias factor. For organizations: require reference class forecasting for any project above a threshold size.