science

Planning fallacy

Planning fallacy is a science topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Planning fallacy rather than just read about it. In short: The planning fallacy is a phenomenon in which predictions about how much time will be needed to complete a future task display an optimism bias and underestimate the time needed. This phenomenon sometimes occurs regardless of the individual's knowledge that past tasks of a similar nature have taken longer to complete than generally planned.

Key takeaways

Planning fallacy belongs to science; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Planning fallacy to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Planning fallacy from memory before moving on to harder problems.

Reference excerpt

The planning fallacy is a phenomenon in which predictions about how much time will be needed to complete a future task display an optimism bias and underestimate the time needed. This phenomenon sometimes occurs regardless of the individual's knowledge that past tasks of a similar nature have taken longer to complete than generally planned. The bias affects predictions only about one's own tasks; when outside observers predict task completion times, they tend to exhibit a pessimistic bias, overestimating the time needed. The planning fallacy involves estimates of task completion time that are more optimistic than the time it took for similar projects in the past. The planning fallacy was first proposed by Daniel Kahneman and Amos Tversky in 1979. In 2003, Lovallo and Kahneman proposed an expanded definition as the tendency to underestimate the time, costs, and risks of future actions and at the same time overestimate the benefits of the same actions. According to this definition, the planning fallacy results in not only time overruns, but also cost overruns and benefit shortfalls.

Empirical evidence

For individual tasks In a 1994 study, 37 psychology students were asked to estimate how long it would take to finish their senior theses. The average estimate was 33.9 days. They also estimated how long it would take "if everything went as well as it possibly could" (averaging 27.4 days) and "if everything went as poorly as it possibly could" (averaging 48.6 days). The average actual completion time was 55.5 days, with about 30% of the students completing their thesis in the amount of time they predicted. Another study asked students to estimate when they would complete their personal academic projects. Specifically, the researchers asked for estimated times by which the students thought it was 50%, 75%, and 99% probable their personal projects would be done.

13% of subjects finished their project by the time they had assigned a 50% probability level; 19% finished by the time assigned a 75% probability level; 45% finished by the time of their 99% probability level. A survey of Canadian tax payers, published in 1997, found that they mailed in their tax forms about a week later than they predicted. They had no misconceptions about their past record of getting forms mailed in, but expected that they would get it done more quickly next time. This illustrates a defining feature of the planning fallacy: that people recognize that their past predictions have been over-optimistic, while insisting that their current predictions are realistic.

For group tasks Carter and colleagues conducted three studies in 2005 that demonstrate empirical support that the planning fallacy also affects predictions concerning group tasks. This research emphasizes the importance of how temporal frames and thoughts of successful completion contribute to the planning fallacy.

… excerpt ends here. Continue reading the full article.

Worked examples

Example 1 — a first encounter with Planning fallacy

Start with the simplest possible case. Write down what Planning fallacy claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Planning fallacy before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Planning fallacy ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Planning fallacy

In research: Planning fallacy appears in science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Planning fallacy in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Planning fallacy is common in secondary-school and first-year university syllabi. It links to neighbouring topics Cognitive biases, Planning, Prospect theory, so understanding it makes those chapters shorter.
In everyday life: Look for Planning fallacy outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Planning fallacy in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Planning fallacy means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Planning fallacy out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Planning fallacy in simple terms?

Why does Planning fallacy matter?

Because it connects several science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Planning fallacy?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Planning fallacy.