The planning fallacy: why everything takes longer than we think

How long will it take you to sort out your paperwork, write a report or paint a room? Most people give an answer that later turns out to be very optimistic. In 1979 the psychologists Daniel Kahneman and Amos Tversky called this tendency the planning fallacy.

The students and their theses

In 1994 the psychologists Roger Buehler, Dale Griffin and Michael Ross asked final-year students to predict when they would finish their theses. On average they predicted about 34 days. In reality the average was about 56 days. Even when asked about the "worst possible case", many did not finish within that time either.

The researchers noticed something interesting. When predicting, the students thought almost only about the scenario of the new project: how they would do it step by step. They rarely recalled how long their previous projects had taken. And when reminded of them, they often explained past delays as special cases that "won't happen again".

Not just students

The same tendency is seen in large public projects. A well-known example is the Sydney Opera House: under the original plan it was to be finished in 1963 for about 7 million Australian dollars; it was completed in 1973 for about 102 million. Researchers in project planning have found similar cost and time overruns in many countries.

We plan as if everything will go well. But life brings phone calls, illnesses and surprises that we did not write into the plan.

The outside view

Kahneman proposed a remedy: the "outside view". Instead of asking "how will I do this?", ask "how long did similar projects usually take, for me or for others?". This method, developed further by the Danish researcher Bent Flyvbjerg, is called reference class forecasting.

Practical advice

1. Keep a record of how long your recurring tasks really take. After a few weeks you will have your own data.
2. Break the project into small steps and estimate each one. Forgotten steps are often the source of delay.
3. Add a buffer of time, for example a third, especially for new kinds of work.
4. Ask someone else. Others often estimate our projects more realistically than we do.
5. Set intermediate deadlines so that you see delays early.