While the planning fallacy is absolutely a problem, the issue is that "roughly similar" is a hard thing to gauge in software development. Unless you're doing f

While the planning fallacy is absolutely a problem, the issue is that "roughly similar" is a hard thing to gauge in software development. Unless you're doing fairly repetitive work (cranking out brochure websites for clients, for instance), you shouldn't be doing any "roughly similar" projects! Each thing you do should be brand new—if it wasn't, you'd just reuse the code you already have. That is the perennial dilemma in software estimation. You don't know how big it is until you're halfway done building it. I'm not proposing a solution to this problem, just pointing out that this IS a problem, and probably not one that can be solved in a one-line bit of advice. Saying "cut it up into pieces" or "estimate similar past projects" is like telling a nervous guy to "just be himself" on a first date.