Why do we underestimate how long it will take to complete a task?

The 

Planning Fallacy

, explained.
Bias

What is the Planning Fallacy?

The planning fallacy describes our tendency to underestimate the amount of time it will take to complete a task, as well as the costs and risks associated with that task—even if it contradicts our experiences.

planning falacy

Where this bias occurs

John, a university student, has a paper due in one week. John has written many papers of a similar length before, and it generally takes him about a week to get it done. Nonetheless, as he is dividing up his time, John is positive that he can finish the assignment in three days, so he puts off starting. In the end, he doesn’t have the paper finished in time, and needs to ask for an extension.

Related Biases

Sources

  1. Ackerman, C. E. (2019, April 7). Pollyanna principle: The psychology of positivity bias. PositivePsychology.com. https://positivepsychology.com/pollyanna-principle/
  2. Hoorens V. (2014) Positivity Bias. In: Michalos A.C. (eds) Encyclopedia of Quality of Life and Well-Being Research. Springer, Dordrecht. https://doi.org/10.1007/978-94-007-0753-5
  3. Lovallo, D., & Kahneman, D. (2003, July). Delusions of success: How optimism undermines executives’ decisions. Harvard Business Review. https://hbr.org/2003/07/delusions-of-success-how-optimism-undermines-executives-decisions
  4. 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(3), 366-381. https://doi.org/10.1037/0022-3514.67.3.366
  5. Kahneman, Daniel; Tversky, Amos (1977). “Intuitive prediction: Biases and corrective procedures” (PDF). Decision Research Technical Report PTR-1042-77-6. In Kahneman, Daniel; Tversky, Amos (1982). “Intuitive prediction: Biases and corrective procedures”. In Kahneman, Daniel; Slovic, Paul; Tversky, Amos (eds.). Judgment Under Uncertainty: Heuristics and Biases. Science. 185. pp. 414–421.
  6. Tversky, A., & Kahneman, D. (1982). Judgment under uncertainty: Heuristics and biases. Judgment under Uncertainty, 3-20. https://doi.org/10.1017/cbo9780511809477.002
  7. Koole, S., & Van’t Spijker, M. (2000). Overcoming the planning fallacy through willpower: Effects of implementation intentions on actual and predicted task-completion times. European Journal of Social Psychology, 30(6), 873-888. https://doi.org/10.1002/1099-0992(200011/12)30:6<873::aid-ejsp22>3.0.co;2-u
  8. Forsyth, D. K., & Burt, C. D. (2008). Allocating time to future tasks: The effect of task segmentation on planning fallacy bias. Memory & Cognition, 36(4), 791-798. https://doi.org/10.3758/mc.36.4.791
  9. Construction begins. (n.d.). Sydney Opera House. https://www.sydneyoperahouse.com/our-story/sydney-opera-house-history/construction-begins.html
  10. Lavallé, O. (2008, March 6). Canadian Pacific railway. The Canadian Encyclopedia. https://www.thecanadianencyclopedia.ca/en/article/canadian-pacific-railway
  11. Optimism bias. (2019, August 22). The Decision Lab. https://thedecisionlab.com/biases/optimism-bias/
  12. Dunning–Kruger effect. (2020, July 22). The Decision Lab. https://thedecisionlab.com/biases/dunning-kruger-effect/
  13. Buehler, R., Griffin, D., & Ross, M. (2002). Inside the planning fallacy: The causes and consequences of optimistic time predictions. Heuristics and biases: The psychology of intuitive judgment, 250-270.
  14. Yamini, S., & Marathe, R. R. (2018). Mathematical model to mitigate planning fallacy and to determine realistic delivery time. IIMB Management Review, 30(3), 242–257. https://doi.org/10.1016/j.iimb.2018.05.003

About the Authors

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Dan Pilat

Dan is a Co-Founder and Managing Director at The Decision Lab. He is a bestselling author of Intention - a book he wrote with Wiley on the mindful application of behavioral science in organizations. Dan has a background in organizational decision making, with a BComm in Decision & Information Systems from McGill University. He has worked on enterprise-level behavioral architecture at TD Securities and BMO Capital Markets, where he advised management on the implementation of systems processing billions of dollars per week. Driven by an appetite for the latest in technology, Dan created a course on business intelligence and lectured at McGill University, and has applied behavioral science to topics such as augmented and virtual reality.

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Dr. Sekoul Krastev

Sekoul is a Co-Founder and Managing Director at The Decision Lab. He is a bestselling author of Intention - a book he wrote with Wiley on the mindful application of behavioral science in organizations. A decision scientist with a PhD in Decision Neuroscience from McGill University, Sekoul's work has been featured in peer-reviewed journals and has been presented at conferences around the world. Sekoul previously advised management on innovation and engagement strategy at The Boston Consulting Group as well as on online media strategy at Google. He has a deep interest in the applications of behavioral science to new technology and has published on these topics in places such as the Huffington Post and Strategy & Business.

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