Last week I took my car in for scheduled service, and when I asked how long it might take, the guy said it was specified as one-and-a-half hours of labor. He could do that because there are thousands upon thousands of data points where a competent mechanic has performed that exact service, so there is a very clear idea how long it takes.
But ask a researcher how long it will take to find what they’re looking for, and the answer is usually, “I have no idea. I’ll know after I find it.” The path a researcher follows is usually new and unexplored, so it’s impossible to predict how long the path actually is.
Creating new software is much more like research than auto service, because it involves traveling unknown ground. Despite this, software development managers often act as if new development is predictable.
It often isn’t!