Hofstadter's law
planning fallacy · it always takes longer
It always takes longer than you expect, even when you take into account Hofstadter's law. Douglas Hofstadter stated it in 1979, and the self-reference is the point: knowing about the bias does not remove it.
In practice
The estimate was three weeks, you added a week for safety, and it took seven. The buffer was calculated from the same optimistic picture as the estimate.
The common mistake
Responding by padding the estimate. A multiplier applied to a number produced by the wrong method inherits the error, which is why doubling estimates works less well than people expect.
Douglas Hofstadter put it in Gödel, Escher, Bach in 1979, and the recursive clause is what makes it more than a joke: it always takes longer than you expect, even when you take into account Hofstadter's law.
Knowing about the bias does not correct it, which is unusual and worth understanding.
Why the buffer does not save you
An estimate is built by imagining the work: these steps, this long each, added up. The picture is of the project going as planned, because that is the only version anyone can picture in detail.
What actually consumes the time is the set of things not in the picture — the dependency that was not ready, the question nobody could answer for three days, the rework after a decision changed. None of these are in the plan because they are not knowable in advance, and there are always some.
Padding the estimate does not help much, because the padding is a percentage of a number generated by the same flawed method. Twenty percent more of an imagined smooth run is still an imagined smooth run.
What works instead
Use the outside view. How long did the last five comparable projects take? That number already contains all the interruptions nobody can predict individually, and it is almost always longer than the bottom-up estimate. When the two disagree, the outside view wins. This is base rate neglect applied to your own calendar.
Estimate in ranges and commit to the top. A range forces the question of what the bad case looks like, which the single number hides.
Decompose only to where you have history. Breaking work into smaller pieces helps when you have measured pieces of that size before, and adds false precision when you have not.
Track your own multiplier. The ratio of actual to estimated is stable per person and per kind of work. Measuring it once gives you a correction factor with evidence behind it, and it is usually larger than anybody wants to admit.
Concept web
Open the full webQuestions
What is Hofstadter's law?
That things always take longer than expected, even when the law itself is taken into account. Douglas Hofstadter stated it in 1979, and the self-referential clause records that awareness of the bias does not remove it.
Why do project estimates always run over?
Because an estimate is built by imagining the work going as planned, which is the only version anyone can picture in detail. The time is consumed by unforeseeable interruptions that are never in the picture and always occur.
Does adding a buffer fix underestimation?
Not much. A percentage buffer is applied to a number produced by the same flawed method, so it inherits the error. Comparing against how long similar past projects actually took works better than padding.