Be Pessimistic to be Realistic!

From numerous researches we know that our tasks rarely become finished within the expected time period. We finish our assignments even later than our imagined worst-case-scenario. Humans fail to implement unforeseen delays or interruptions in their planning. This can happen to any of us: No matter whether you are writing an essay or planning a megaproject.

However, researchers also showed that outsiders are able to estimate the time required for a given task much more accurately than the people involved. As an outsider, we use experiences from similar tasks to arrive at much more reasonable time frames, although these sometimes seem to be ridiculously long. Relying on the method of viewing our planning as an objective outsider helps us to overcome this bias – and to get one step closer to our goal of rational thinking.

In the second trimester of my law studies, I had to submit an assignment in State Church Law. At least three thousand words and it was compulsory for passing the course. Since I liked both the class and the lecturer, I decided to submit a thoroughly researched and well-structured essay. Planning all steps, I had in mind ten hours of research, five hours of developing a good structure and about 13 hours of further research, drafting and finalising the essay. I planned to finish it by Saturday evening. In fact, I was done on Sunday, 11:45 pm: 15 Minutes before the deadline.

Without being even aware of it, I fell prey to the Planning Fallacy. This being a classic example most of us went through and will go through, e.g. when it comes to writing an essay or even our bachelor thesis. In the following, I will explain this fallacy and provide you a few possible solutions for you to avoid being trapped yourself.

Understanding the Research

In 1994, Buehler and colleagues asked psychology students how much time they expected to need to finish their senior theses. The average estimate was 33.9 days, while they expected an average of 27.4 days if everything “went as well as possible”. On the other hand, if everything went the worst way, they expected to complete the task in 48.6 days. It will may surprise you that only 30 % of the students finished their thesis by the day they had predicted. Further you might find it interesting that the completion took an average time of 55.5 days – even more time than under the worst-case-prediction (Buehler, Griffin & Ross, 1994).

The same researchers conducted another study the year after. This time, they asked students to estimate the probability they complete their personal assignments by a time with 50 %, 75 % and 99 %. Only 13 % finished by the 50 %-probability-mark, 19 % by the time of 75 % and only 45 % by the time of the 99 % expectation (Buehler, Griffin & Ross, 1995).

Newby-Clark and colleagues found that participants who were asked for their plans based on realistic scenarios estimated a time frame that was indistinguishable from the time frame that they expected in best-case-scenarios.

Finding a Solution

Let us focus on the possible solutions for the Planning Fallacy. You might say “Oh, that’s not a problem. I will simply not be part of the group with bad self-assessment; namely 70 % of the first experiment or the 55 % of the second”. Since Rational Thinking is mostly based on data and probability, and since the odds are definitely set against you, this would not at all be a rational way to protect yourself against the Planning Fallacy.

Worse still, you might discover that simply trying to plan as accurately as possible makes your plan even more vulnerable to the Planning Fallacy. The more details are implemented in a plan, the more optimistic it becomes (for additional information on this counterintuitive finding, click here).

Likewise, it would not be wise to simply add a specified time period to your expectation. Why so? On the one hand it would probably be inaccurate since it is a mere guess. This guess will lead you to the time of your best-case-scenario with the addition of an unreasonable amount time. On the other hand, remind yourself of Parkinson’s Law, saying that “work expands so as to fill the time available for its completion”. You know this from writing an assignment: You make last minute improvements or changes that are not necessary (see also this piece on The Economist).

Eliezer Yudkowsky suggests, as many other authors alike, to use an “Outside View” instead of an “Inside View”. The outside view is protecting your mind from all the details of your case at hand and is just taking into consideration how long it took you to finish roughly similar cases before. You might find it amusing that a similar result can be expected from asking experienced third persons that do not know the details of your case. In fact, Bazerman and Moore recommend asking, for instance, a trusted friend who has experience with similar situations (2013). Both ways will lead to expectations that have been proven highly accurate though they seem to be ridiculously long. However, this is the best way for you to get reliable data for your plans. Remember Buehler’s experiment: You should expect to need a long time to complete your task since the average completion time is likely to be even worse than your worst-case-scenario.

Therefore: Plan pessimistic to plan realistic!


Having read all this, you now have guidelines to plan your tasks more accurately. You have some knowledge about the Planning Fallacy and therefore the opportunity to overcome this bias.

Applied to my case, I know that it took me about 130% of the expected time to finish a three thousand words assignment. Using these data, I will set 38 hours for my next comparable essay. It is of utmost importance that I do not simply jump to all all the reasons for which it could take less time. I paid more attention in class? Irrelevant! I did prepare more thorough notes? Nonsense! I just use the data of the last essay. Thereon I will build my plans – this is the best way to draw near rationality.


Philip J. Hattemer is in his second year at Bucerius Law School. Before University, he has been trained to be a Hypnotist and partly trained in Neurolinguistic Programming. In this time his penchant for psychology led him into Rational Thinking.

(Photo courtesy of pixabay)