"It only takes fifteen minutes"

Start with the estimate, because everything else rests on it. The walk to the meeting takes fifteen minutes. That's what you'd say if asked, and you'd say it with confidence.

Fifteen minutes is what the walk takes on a day when the keys are in the right coat, the path is dry, you don't meet anyone, the lift is waiting, and nobody stops you on the stairs. It's the time from pavement to pavement, on a good day, at a brisk pace. It isn't the time from sitting at your desk to sitting in the meeting room. That includes finding things, putting on a coat, locking the door, the stairs at both ends, and the corridor. And it isn't an average. It's a best case.

Roger Buehler, Dale Griffin and Michael Ross gave this pattern a name in 1994. They called it the planning fallacy, and it's the tendency people have to underestimate how long their own tasks will take. In their studies, people predicted when they'd finish academic and everyday projects, and were too optimistic across the board. When the researchers asked people to think aloud while making their predictions, almost all of the thinking was about the plan. People described how the task would go, step by step, and the steps went well. Very little of the thinking drew on how long similar tasks had taken in the past.

That's the mechanism, and I think it fits the walk exactly. When you say fifteen minutes, you're running a short film of the walk in your head, and the film shows the walk going well. Films of walks don't include hunting for keys. They don't include the wet path. The film is a plan, and the plan is a best case, and you read the number off the plan.

There's a further finding that explains why the plan doesn't correct itself. A later study by Ian Newby-Clark and colleagues asked people to imagine pessimistic versions of how their task might go, as well as optimistic ones. Imagining things going wrong didn't change people's predictions. And when asked, people consistently rated the pessimistic versions as less plausible than the optimistic ones. From inside your own head, the version where you find your keys and the lift is waiting feels like what will happen, and the version where three small things go wrong feels like bad luck, even though three small things go wrong most days.

So the estimate has a built-in lean. It's not that you're bad at judging time. It's that the method you're using, which is imagining the trip, can only produce a good-day number. Your arrival times then lean the same way, late and never early, because the truth is scattered around the average and you've been aiming at the best case.

Think of an estimate as having two halves. There's the part you can picture, and there's the part you can't, which is made of the things that will go slightly wrong. The picture gives you fifteen minutes. The other half is real, and it's invisible, and you can only get at it another way. Chapter 16 will show you how.