55.5 Days: What a Thesis Study Says About Your Goal Deadlines

Thirty-seven psychology honors students were asked, for a study published in 1994, to guess how long their theses would take. The average guess was 33.9 days. The average thesis took 55.5 days. Only about 30 percent finished by the date they had predicted, and fewer than half made it even by the date they gave when asked for the worst case, which averaged 48.6 days (Buehler, Griffin and Ross, Journal of Personality and Social Psychology, 1994). So the short answer to why your goals take longer than planned: people plan from the best-case story in their head and ignore how similar projects have actually gone for them.

That habit has a name. The planning fallacy is the tendency to underestimate how long a task will take even when you know that similar tasks ran late before. Daniel Kahneman and Amos Tversky introduced the idea in 1979, and the 1994 paper is the study most people cite as the experimental proof.

What did the thesis study actually measure?

Buehler and colleagues asked students for three numbers: a realistic finish date, a best-case date, and a worst-case date. They then waited and recorded when the theses were handed in.

The part that matters for anyone with a goal is the worst-case number. The students were not being careless. They were asked to imagine everything going wrong, and reality was still slower. A worst-case scenario built from the inside is still a story about this project, with its own plan, and it leaves out the stuff nobody plans for: the flu, the broken laptop, the three weeks when the work just felt bad.

The same paper reports that people underestimate their own completion times but not other people’s. When you judge someone else, you use their track record. When you judge yourself, you use your intentions.

Is the Sydney Opera House the same mistake at scale?

It is the classic example, and the numbers are public. The Sydney Opera House was budgeted at 7 million Australian dollars, construction began in 1959, and it opened in 1973 at a reported cost of 102 million, about fourteen and a half times the estimate. Accounts of the project say it opened roughly ten years later than first planned. Bent Flyvbjerg, the Oxford researcher who has spent decades building databases of project overruns, uses it as a standing example of a megaproject estimate that was far from what happened.

One caveat keeps this honest. Some accounts describe the original 7 million figure as a politically convenient low number, set to get the project approved before an election. That is a different mechanism from the psychological bias in the thesis study, and in real projects the two usually mix. For a personal goal, though, nobody is lobbying you. You are simply lowballing yourself, and nobody checks.

What is the outside view, and how does it fix the estimate?

Kahneman and Tversky separated two ways to forecast. The inside view builds the forecast from the details of this project: here are my steps, here is how long each takes. The outside view ignores the details and asks a blunter question: of all the projects like this one, how long did they take?

Flyvbjerg later turned the second approach into a working method called reference class forecasting. You pick a class of comparable past projects, look at the distribution of how they actually went, and place your project inside that distribution. The idea was developed for bridges and railways, but the logic scales down. For a personal goal, the reference class is your own history plus anyone else’s who tried something similar.

How do you use a multiply-by number on your own goal?

Here is the version I would actually run, on paper, in ten minutes.

1. List three or more past projects of the same kind. Not your dreams, the real ones: the report, the decluttering, the course you bought.
2. Write the planned duration and the real one for each. Calendars and file dates make this easier than memory does.
3. Divide actual by planned. That gives each project’s ratio.
4. Take the middle ratio. Use the median so a single catastrophe does not run the show.
5. Multiply your new plan by it. The result is your schedule. Keep your original date as the hoped-for one.

The thesis study gives a sample ratio: 55.5 divided by 33.9 is about 1.64. Your own number could be lower or higher.

A worked example

Say the goal is a 40,000-word draft of a book, and your gut says 90 days. You pull three past projects:

Project	Planned	Actual	Ratio
Work report	2 weeks	3 weeks	1.5
Course notes	4 weeks	6.5 weeks	1.6
Garage clear-out	10 days	18 days	1.8

The median ratio is 1.6. Ninety days times 1.6 is 144 days, about 4.7 months. That is the schedule you write at the top of the page.

The reason this matters has less to do with accuracy than with what you do on day 60. On the 90-day plan, day 60 should put you at two thirds, about 27,000 words. You are at 17,000 and the thought arrives that you are behind and always are. On the 144-day plan, 17,000 words at day 60 is close to on schedule. Same pace, same person, different reaction. A schedule that is wrong from the start turns ordinary work into a series of small failures, and people quit when they feel the failure, not when the work gets hard. The same pattern shows up in why people keep restarting instead of finishing and in what happens when a goal gets left alone for a month.

Why does a drifting deadline kill goals?

A deadline that sits months away gives you nothing to check against. The slip happens in increments of a day here and a weekend there, and each one is small enough to ignore. By the time the gap is visible it is large, and the feeling that arrives is shame, not information.

George Doran’s 1981 paper that gave us SMART goals asked for a time-bound target, and what Doran actually wrote was written for managers, and a solo goal has no manager checking the date. The fix is to attach the long goal to a short, checkable unit.

A wake-up time is about the cleanest one available. It is a deadline that arrives every day, it is binary, and it cannot be renegotiated after the fact. If the goal is 144 days of writing from 6:30 a.m., you know by 6:31 whether today happened. A 31-day log of holding one wake time shows how small and measurable that daily unit is. DontSnooze, the alarm app from dontsnooze.io, is built for exactly this: you record a short video within 30 seconds of the alarm, and if you do not, a random photo from your camera roll goes to friends you picked. That makes the daily checkpoint one you cannot quietly skip, which is the gap the planning fallacy opens in every long schedule.

Limits apply. A wake-time commitment tells you the day started on time, not that the work got done, so the multiply-by number still has to carry the long-range estimate. The thesis study also used a small sample of students on one kind of task, and the 1.64 ratio should be read as an example, not a law.

Quick recap of the method

Plan the way a stranger would plan for you. Pull your own past ratios, multiply, write the longer date down, and put one daily check beside it so the slip shows up on the day it happens. A goal that takes 144 days was never a 90-day goal that failed.