Why Your Project Estimate Is Wrong Before You Start

You said the redesign would take a weekend. It's been three. Different project, same math, same surprise — you've done this to yourself before. Psychologists have known about it since 1994, and they gave it a name. The fix isn't estimating more carefully. It's estimating differently.

The 1994 study that named it

In 1994, psychologists Roger Buehler, Dale Griffin, and Michael Ross asked a group of students one question: when will you finish your thesis? Average answer: 33.9 days. Average actual completion: 55.5 days, about two-thirds longer than predicted. Even when they asked for a pessimistic, worst-case guess, the real world beat it most of the time.

Here's the part that matters for how you plan: it wasn't that people forgot how long things usually take them. Buehler's team found something sharper. People remembered their past overruns just fine. They just decided those didn't apply here. This project is simpler. This time you've cleared your schedule. This one's different. That reasoning has a name: the inside view. You imagine the plan executing cleanly, step by step, and build your estimate from that imagined movie instead of from what actually happened the last three times.

There's an alternative. Instead of asking "how do I picture this one going?" you ask "how long did the last few things like this actually take?" Researchers call it the outside view. It sounds almost too simple to be a fix. It is the fix.

ADHD stacks a second bias on top of the first

The planning fallacy shows up in anyone. Those thesis-writing students didn't have anything wrong with their attention, and they were still off by weeks. But if you have ADHD, you're fighting that same universal bias with a warped ruler underneath it.

A 2021 meta-analysis in the Journal of the American Academy of Child & Adolescent Psychiatry, led by Marx and colleagues, pooled 55 studies and thousands of participants testing how accurately people judge, produce, and reproduce stretches of time. Across the board, ADHD came out with real, measurable impairment: not a vague sense that time feels off, but an actual, quantifiable gap, showing up in tasks as simple as pressing a button to mark ten seconds. The deficit showed up whether people were comparing two durations, estimating one, or trying to reproduce one they'd just experienced.

Put the two findings together and the picture gets specific. Everyone defaults to the inside view and lowballs the result. ADHD brains run that same optimistic imagining through a timing system that's already unreliable. The gap between guess and reality tends to run wider — not because you're worse at planning, but because you're compounding a universal bias with a real perceptual one.

The billion-dollar fix, shrunk down

Governments got tired of losing this exact bet. The Sydney Opera House is the textbook case: budgeted for a few years and a few million dollars, it took over a decade and ran wildly past its budget, for the same reason your redesign did — planners imagined the ideal execution instead of consulting the record. Economist Daniel Kahneman's answer, now used by transportation authorities in the UK and Denmark, is called reference class forecasting. Strip out the infrastructure jargon and it's three moves: name a handful of past projects genuinely similar to this one, look at what they actually took (not how you remember them feeling), and set your estimate from that data instead of from imagining this project fresh.

You can run the same three moves on a Tuesday afternoon without a consulting firm. Before you commit to a timeline for anything (a work deliverable, a house project, that "quick" errand list), stop and name two or three past things that were genuinely similar in shape. Not the same subject. The same shape — how many moving pieces it has, how much coordinating with other people it takes. Write down how long those actually took, start to finish, not how fast they felt once you were deep in them. Skip your gut. Skip the version where nothing goes wrong. That number is your starting estimate.

This works precisely because it routes around the part of your brain that's bad at this job. You're not being asked to imagine harder or plan more thoroughly; imagining is exactly the skill that keeps failing you here. You're being asked to look something up, the way you'd check a calendar instead of trying to remember what day it is.

Build the reference class before you need it

The habit gets easier the more of a track record you're sitting on, so start collecting now, even loosely. When something wraps up, jot one line: what it was, what you guessed, what it actually took. You don't need a system for this. A note in your phone works. Give it a month and you'll have your own personal dataset, built from your actual patterns instead of anyone's generic advice about ADHD and time.

Next time you're about to promise a weekend for something that isn't one, pull that list instead of your gut. You'll probably still be off. But you'll be off by less, and off in a direction you can plan around instead of getting blindsided by every single time.

This is general information, not medical advice. If ADHD is affecting your daily life, a qualified professional can help you find what actually works for you.

References

• Exploring the "Planning Fallacy": Why People Underestimate Their Task Completion Times — Buehler, Griffin & Ross, Journal of Personality and Social Psychology (1994)
• Meta-Analysis: Altered Perceptual Timing Abilities in Attention-Deficit/Hyperactivity Disorder — Marx et al., Journal of the American Academy of Child & Adolescent Psychiatry (2021)
• Reference class forecasting: promises, problems, and a research agenda moving forward — Cantarelli et al. (2025)