Planning Fallacy

Explanations > Theories > Planning Fallacy

Description | Research | Example | So What? | See also | References 

Description

When building plans, we tend to under-estimate how long things will take in practice.

This is often due to an assumption that no risks will occur, no changes will happen to the specification, nobody will be ill or leave, all supplies will arrive on time and so on.

A common rule of thumb for project managers is to do a best-case estimate then multiply by two or more, depending on the project.

Research

Buehler et al asked students to estimate how long a thesis would take to complete. The average response was 33.9 days. In practice, they took an average of 55.5 days.

Example

In planning a business change project I assume that everyone will cooperate and attend all meetings and not object to any of the changes. Of course this does not happen as expected and the subsequent issues make the project drag on for three times as long as I had expected.

So What?

Using it

When making plans, multiply your first estimate by at least two. When others make plans, check that they have plenty of contingency in and have planned to manage the risks.

See also

Neglect of probability bias, Focalism

References

Buehler, Griffin & Ross (1994)