Authors can learn a lot from Daniel Kahneman’s book, Thinking, Fast and Slow. A psychologist and Nobel-prize winner, Kahneman points to flaws in human thinking. One of them is the “planning fallacy.” Simply put: Most things take a lot longer to finish than we think, because we don’t think rationally.

Kahneman tells how get gathered a bunch of people together to write a textbook. He asked everyone to write down how long he or she thought the project would take. The answers ranged from one and a half to two and a half years. Then he asked an expert in the group to speak from experience with comparable projects. The expert replied, a bit sheepishly: seven to ten years.

Sheepishly because he was demonstrating the bit of irrationality that Kahneman intended: The expert himself had written down two years just moments before.

What’s irrational is that, as humans, we all estimate time demands based on a sampling of work we have done so far on a specific project. We meanwhile forget to factor in either our own previous experience or data from elsewhere. We usually forget to even ask for outside data.

You might think this sounds like ox-like stupidity. How could Kahneman’s expert not have associated past projects with the present one? What explains this departure from common sense? Kahneman says simply: It’s human. We all do it. Even he does it, he admits, although he has learned to catch himself.

What’s the lesson when you’re writing a book? Don’t treat the current case like a special case. Don’t be like everyone else! Every time you start a project, work step by step through a schedule. Don’t add up all the building blocks and then, irrationally, say, “It can’t possibly take that long!” The truth is that it can. And it does.

The planning fallacy has a strong grip on us. We can’t really shake it. A serious book almost always takes well more than a year. So the question is, what is your factor of underestimation? Let Kahneman’s experience be your guide: Like everyone else in his textbook group, he underestimated the time needed by a factor of four—the textbook took eight years!

When you do your next schedule, don’t jump to overly optimistic dates for finishing. Ask: Do I need to multiply by four?

[Revised January 2020. Originally published October 25, 2013]