principlebeginnerconcrete

Planning Fallacy

Systematic tendency to underestimate how long tasks will take and what can go wrong, despite past experience with similar projects.

In Depth

People make optimistic predictions about their own projects while correctly predicting that similar projects by others will face delays and problems. This occurs because we focus on the specific plan (inside view) rather than the statistical distribution of outcomes for similar projects (outside view). The planning fallacy combines optimism bias, anchoring on best-case scenarios, and failure to consider what typically goes wrong. Even experienced professionals fall victim when planning their own projects.

Example

A software team estimates a project will take three months based on their plan, ignoring that their last five projects took twice as long as estimated. They focus on their specific plan rather than the base rate of past performance.

Common Misconception

Detailed planning prevents delays—detailed plans often increase confidence without improving accuracy, because they don't account for unknown unknowns.

Source

Thinking, Fast and Slow

Daniel Kahneman

project managementforecastingdecision-making

Related Concepts

builds on

Pre-Mortem

Before starting a project, assume it has failed and work backward to identify what went wrong, surfacing risks that optimistic planning overlooks.

Test Your Understanding

active recallmedium

Why do people make optimistic predictions about their own projects while correctly predicting delays in similar projects by others?

concept connectionhard

How does the planning fallacy relate to the 'Plateau of Latent Potential' concept from Atomic Habits?