What do I need to know about the planning fallacy?

The Irrational Tech: Part 1

Julia Clavien


This is a series. Find Part 0 here.


Everyone was in the room — the devs, QA, analysts, PMs and more. I looked over the plans and confidently suggested that given our velocity we would hit our date. There were murmurs of agreement around the room. I felt confident, we had a great team, and there was contingency.

Yes, on other projects in the past things had taken longer than expected, but this one was “different”…

Well I’m sure you can see where this going.

The client was not impressed when delivery took twice as long as planned.

The Planning Fallacy

The Planning Fallacy relates to our tendency to systematically underestimate the time & resources required to complete projects.

Is that really the mostly likely date we will be finished? Or is some kind of Planning Fallacy and Optimism Bias at play?

Psychologists Daniel Kahneman and Amos Tversky first studied and identified the Planning Fallacy. They found it to be not only rife, but very hard to avoid, even with great experience this fallacy can catch out humans over and over again.

The very aptly named Chaos report last year reported that “fewer than a third of all projects were successfully completed on time and on budget over the past year”.

There are loads of stats around software development estimates, but in suffice to say, very few projects (whatever the methodology) don’t have some kinds of cost and schedule overrun.

Why do we get it so wrong?



Julia Clavien

Curious to a fault. Technology | Psychology | Philosophy. All opinion subject to change. ☺ linktr.ee/juliaclavien