CODEXCard 4 of 8 · Core Cognitive Biases
Planning fallacy
Every plan is the best case.
The planning fallacy is the reliable gap between how long we expect a task to take and how long it really takes. Kahneman and Tversky named it in 1979, and it applies to kitchen renovations, software releases, essays and public infrastructure alike. Estimates come out too short and too cheap, again and again.
It happens because planning is an act of imagination. You build a story of the work going step by step, and that story naturally leaves out the illness, the delayed supplier, the bug nobody saw coming. Kahneman calls this the inside view. It feels detailed and specific, which makes it feel accurate, even though it ignores everything that usually goes wrong.
A tempting fix is to add a standard buffer, say twenty percent, to every plan. That helps a little but misses the point, because the real error can be much larger and varies by type of project. The stronger method is the outside view, sometimes called reference class forecasting: find similar past projects, see how long they actually took, and start from that.
The planning fallacy pairs naturally with a margin of safety. Once you know your plans run optimistic, building slack into deadlines and budgets stops feeling like pessimism and starts looking like realism.
Example
You decide to repaint the spare bedroom on a Saturday morning and expect to be done by lunch. You forget moving furniture, taping edges, a second coat and the trip back to the shop for more paint. Remembering that your last two painting jobs each took a full weekend gives a far better estimate.
Try it this week
For the next task you schedule, write down how long the last similar task actually took and plan from that number.
From Core Cognitive Biases (Daniel Kahneman · 2011).
