Here you go again - frantically working to beat the deadline for a project you thought would be done long ago. Why didn't you see this coming? Why do you repeatedly underestimate how long it will take you to get things done?

This familiar experience, known as the planning fallacy, has been explored by social psychologists since the term was coined by Daniel Kahneman and Amos Tversky in 1979.  The planning fallacy refers to an optimistic prediction bias in which people underestimate the time it will take them to complete a task, despite knowing that similar tasks have typically taken them much longer in the past. An intriguing aspect of the planning fallacy is that people simultaneously hold optimistic expectations concerning a specific future task along with more realistic beliefs concerning how long it has taken them to get things done in the past. When it comes to plans and predictions, people can know the past well and yet be doomed to repeat it.

The best evidence for the planning fallacy comes from studies that follow people into the future. In such studies, people predict how long it will take them to complete an upcoming project and also report how long it has taken them to complete very similar projects in the past. Finally, they carry out the project and report exactly when they completed it.

Typically, participants in these studies exhibit the planning fallacy. For example, university students typically acknowledge that they have typically finished past assignments very close to their deadlines, yet they insist that they will finish the next project well ahead of the new deadline. Then, predictably, they go on to finish the next project (as usual) right at the deadline.

The planning fallacy is remarkably robust. It appears for small tasks like daily household chores (such as cleaning), as well as for large scale infrastructure projects such as building subways.  It generalizes across individual differences in personality and culture, and it applies both to group and individual projects. For example, conscientious people often get things done well before procrastinators, but both groups typically underestimate how long it will take them to get things done.

So why do people repeatedly underestimate how long tasks will take? Why don't they learn from past experience and adjust their estimates accordingly?

Kahneman and Tversky proposed that, as people think about how they will complete a task, they are inclined to take an "inside view" in which they focus on the specifics of the task at hand, paying special attention to its unique features. For example, people imagine and plan out the specific steps they will take to carry out the target project. The problem is that events usually don't unfold exactly as people imagine. Even when people create a thoughtful mental scenario in advance, they will likely encounter unexpected obstacles, delays, and interruptions. People could usually make more realistic predictions by taking an "outside view" in which they base their predictions on their prior experiences. However, people typically overlook this approach, in part, because they feel that their previous experiences are not relevant to the new task.

Dozens of studies have supported the inside-outside explanation of the planning fallacy. They have also taken research on the planning fallacy in new directions. Researchers have come up with strategies that can help to reduce or avoid the planning fallacy. Here are three types of strategies that may help people make more realistic predictions for upcoming projects.

1. Take the outside view.

Several interventions aim to overcome people's failure to base their predictions on historical precedent. For example, "reference class forecasting" is a step-by-step procedure that involves comparing the current project with similar projects the person has completed in the past. This technique requires the person to identify a set of similar tasks they’ve worked on in the past, to plot out how long each task took, and then to compare the task they currently hope to complete with all the past outcomes. This painstaking approach allows them to estimate the most likely outcome this time. And, most importantly, this procedure has been shown to improve forecast accuracy for large scale construction projects and may have similar benefits among people trying to make predictions about personal projects.  Of course, the time it takes people to use this labor-intensive technique may not always be worth the investment of time and energy.  This technique may be more useful to subway builders than to subway sandwich makers. 

2. Alter the inside view.

Given that people often base their predictions on an idealized and optimistic scenario for the task at hand, researchers have developed interventions that prompt people to think “outside the box” as they form their plans. Specific strategies include decomposing the plan into smaller steps (unpacking), generating the plan in reverse-chronological order (backward planning), and visualizing the plan from the perspective of an observer (third-person imagery). Each of these strategies can help people focus less narrowly on their hopes and plans and to consider factors that might delay their progress – such as potential obstacles, injuries, temptations, distractions, interruptions, and competing offers on how to use one’s time.

3. Make plans and predictions that influence behavior.

An alternative path to more accurate predictions is to generate plans and predictions that are strongly binding.  For example, getting people to form "implementation intentions" at the time of prediction – making them  commit to performing  parts of the task at specific times and on specific dates – makes people more likely to carry out those actions, and thus less prone to the planning fallacy.  This is why I gave myself a separate deadline for writing each section of this blog. 

We have learned a great deal about the planning fallacy in the past two decades. However, if I were to predict that future progress on this intriguing bias will be faster than past progress, you’d be right to call me on it. So I’ll just say that I hope that our continued research on this problem will continue to yield new insights – and perhaps help you arrive at more realistic plans and predictions for your next major project.

For Further Reading:

Buehler, R., Griffin, D., & Peetz, J. (2010). The planning fallacy: Cognitive, motivational, and social origins. In M. P. Zanna & J. M. Olson (Eds.), Advances in experimental social psychology (Vol. 43, pp. 1-62). San Diego, CA: Academic Press.

Buehler, R., Griffin, D., & Ross, M. (1994). Exploring the "planning fallacy": Why people underestimate their task completion times. Journal of Personality and Social Psychology, 67, 366-381.   

Flyvbjerg, B., Garbuio, M., & Lovallo, D. (2009). Delusion and deception in large infrastructure projects: Two models for explaining and preventing executive disaster. California Management Review, 51, 170-193.

Kahneman, D., & Tversky, A. (1979). Intuitive prediction: Biases and corrective procedures. TIMS Studies in Management Science, 12, 313-327.


Roger Buehler is a social psychologist who studies the planning fallacy and thought he’d be done with this research long ago.