The Planning Fallacy and Software


The Overcoming Bias blog had a great post on the Planning Fallacy a few days ago. They’ve got some great psych experiments to back the term up, but it boils down to people being inherently bad at figuring out how long a task will take. We always underestimate!

This may sound familiar to anyone who’s worked in software. Guy Kawasaki’s rule of thumb is add six months to the worst case shipping date you’re given, and that sounds right to me. What’s interesting about the post is it not only documents the problem, they also offer a solution.

They describe the normal way people create time estimates as inside planning. This is where you look at the tasks you need to do, create estimates for each of them, and total them to get the final estimate. There’s no time for the tasks you forget, or anything unexpected. What’s interesting is that even when asked to produce a worst-case estimate, people don’t allow enough time.

Their solution is to use outside planning. For this you ignore the unique aspects of the project, and instead try to find a similar completed project, and look at how long that actually took. This usually produces a far more realistic estimate.

This sounds right to me, one of the strengths of an experienced team is that they have a lot of previous projects to compare against. It’s a very strong argument to say ‘well, we’re barely at alpha, and it took us six months to get from here to shipping on project X, so we need to rethink releasing in two months’. If both the team and management were involved in project X, it’s hard to ignore that.

Of course, one of the strengths of a greenhorn team is that they aren’t as cautious! They’re likely to over-commit, but with smart management scaling back the tasks once reality kicks in, they might still produce a better project overall.

Occasionally too, the Captain Kirk/Scotty management/engineering dynamic actually works; "Captain, it will take two weeks to fix!" / "Scotty, get it done in the next thirty minutes." Sometimes that pressure will force a rethink on the engineering side about how to fix something. Maybe there’s a quick hack that will solve 80% of the problem?

Funhouse Photo User Count: 885 total, 72 active daily. I think growth’s actually slowed since my recent changes, so I’ll definitely need to instrument and analyze some usage statistics to try and work out why, and also take a fresh look at the user experience.

Google Hot Keys Download Count: I’ll be occasionally showing my GHK download numbers too, since they’re growing pretty well. I’m up to 4894 total so far on the main Mozilla site. I’m not seeing many IE downloads from my own site, it’s not been approved for CNET downloads yet, and isn’t in any other distribution channels.

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