The Planning Fallacy

I have been reading Less Wrong

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The other night in the pub I was sounding off about the Planning Fallacy.

An experiment was done where people were asked to estimate the time needed to complete a task. They were asked for estimates for the best case, worst case and normal case times.

The experimenters found no real difference between the best case and normal case estimates.

When later, the actual time to do the task was measured, it was, on average, worse than the worse case estimate.

The experimenters drew the conclusion that people expect that usually, everything will go as well as it can possibly go, and that people are spectacularly bad at imagining just how wrong things can go, and how normal it is for them to go that wrong.



The following morning, we were due to go to a cricket match at Newton, which I guessed was about a 20 minute drive from my house. We had planned to meet up in the excellent Queen's Head pub at 12:00. I was giving two friends, Steve and Nick, a lift.

Being a careful person who hates being late, I told them to arrive at my house at 11:30.

At about 11:00, Beard rang to ask if he too could have a lift.

I put my bike and my cricket kit in the van early on so I wouldn't have to worry about it later.

At about 11:30, Steve turned up, while I was still loading the van. He needed a bottle of milk to take to the game. I sent him to the nearest local shop and I put the kettle on, and when he came back,we sat in the back garden while we waited for the others.

Beard and Nick showed up while we were in the garden. We finished our tea, and then loaded their cricket kit into the van, and locked Beard's bike up in my shed.

As we set off, we realised that the main road near my house was blocked by water main repairs, but we quickly worked out an alternative route.

There was little traffic, and the drive to Newton seemed to take about 20 minutes. We had to wait a few minutes at a level crossing for the London train to go through.

Absolutely nothing had gone wrong. We were only about half an hour late. We were the first ones there.