Just ask anyone how long their next project will take and you will get a systematically optimistic answer. The bathroom renovation will be finished in two weeks. The report will be ready on Friday. The move will take one morning. Inevitably, two weeks become two months, Friday slips to the following Tuesday, and the move takes up three full weekends.

This is not a matter of bad faith or laziness. It is a documented cognitive mechanism that no one escapes — not even experts, not even those who know they are vulnerable to it. Psychologists Daniel Kahneman and Amos Tversky named this phenomenon in 1979: the planning fallacy. Since then, decades of research have only confirmed its universality.

A strange bias: knowing is not enough to correct it

What makes the planning fallacy especially fascinating — and frustrating — is that it resists knowledge. One might think that someone who has already missed ten deadlines in a row would eventually revise their estimates upward. In practice, no.

Kahneman and Tversky showed that individuals continue to underestimate the duration of a task even when they have direct experience with similar projects that took longer than expected. At the moment of planning, the brain focuses on the task as it imagines it in the best-case scenario — without integrating unexpected events, interruptions, dependencies on other people, moments of blockage, or simply fatigue.

This is what Kahneman calls the inside view: we project ourselves into the ideal scenario of the project and forget to consult the outside view — that is, what history actually tells us about projects of this type.

55 days instead of 30: a revealing study

An often-cited experiment in this field involved students who were asked to estimate how long they would need to finish their thesis. On average, they predicted about 30 days. Reality? Most finished after 55 days, almost double the estimated time. And only 30% of students managed to meet their own deadline.

This figure deserves reflection: 70% of people fail to meet a deadline they themselves set, fully aware of what they were doing.

This phenomenon is not limited to overwhelmed students. It affects experienced project managers, architects, engineers, doctors, lawyers — everyone. Competence sometimes reduces the gap, but it does not erase it.

Examples on the scale of nations

The planning fallacy does not merely disrupt our personal schedules. It has consequences on an entirely different scale.

The Sydney Opera House is perhaps the most frequently cited example in project management manuals. Designed by Danish architect Jørn Utzon, it was supposed to be delivered in 1963 with a budget of 7 million Australian dollars. It was finally inaugurated in 1973 — ten years late — at a final cost of 102 million dollars, about fourteen times the initial budget. The reasons are classic: incomplete design at launch, underestimated complexity, changes along the way, and technical surprises on the construction site.

This kind of gap is not exceptional in major infrastructure projects. Researcher Bent Flyvbjerg analyzed several hundred major public projects around the world and found that nine out of ten exceed their initial budget.

Why the brain does this

Several cognitive mechanisms combine to produce this bias.

The first is natural optimism: our brain is wired to project positively into the future. This is not a flaw — it is probably what pushes us to start worthwhile projects. But when it comes to estimating, this optimism becomes a blind spot.

The second is focus on the nominal scenario: when we plan, we visualize the steps unfolding smoothly, not the iterations, corrections, or back-and-forth with slow-to-respond contacts. We imagine the most direct path, while reality rarely looks like a direct path.

The third is forgetting past experiences: even if we have lived through the same situation several times, the brain tends to treat each new project as something unique, uncontaminated by previous failures. “This time is different” is an inner phrase many people repeat before watching the deadline slip.

What can we do in practice?

Kahneman himself proposed a simple method: reference to the class of similar projects, sometimes called reference class forecasting. Rather than estimating the time of a project from the inside, by imagining its steps, we start from historical data on comparable projects: how long did they actually take?

This approach is counterintuitive because it forces us to ignore the specific features of our own project and refer to an external comparison base. But precisely this movement — leaving the inside view to adopt the outside view — is what makes it possible to correct the bias.

Another pragmatic approach is to apply a systematic multiplier to one’s estimates. Depending on the type of project and the person’s profile, this multiplier generally varies between 1.5 and 2.5. In other words: if you think a task will take two weeks, plan three to five weeks.

This is not pessimism. It is lucidity.

A flaw everyone has, but no one attributes to themselves

There is one final irony in this bias: when people are asked about their tendency to estimate time poorly, the vast majority admit it is a very widespread problem… among others. They, of course, make reasonable estimates.

This dissonance illustrates the deep nature of the planning fallacy: it does not disappear through awareness. It must be opposed with deliberate mechanisms — data, reference frames, multipliers — if we hope to fall for it a little less often.

In the meantime, the next time you tell yourself that this project will go quickly, take a moment to ask whether you have said that before.