Agile Estimating and Planning by Mike Cohn

Techniques for Estimating " Prediction is very difficult, especially about the future. " —Niels Bohr, Danish physicist The more effort we put into something, the better the result. Right? Perhaps, but we often need to expend just a fraction of that effort to get adequate results. For example, my car is dirty, and I need to wash it. If I wash it myself, I'll spend about an hour on it, which will be enough to wash the exterior, vacuum the interior, and clean the windows. For a one-hour investment, I'll have a fairly clean car. On the other hand, I could call a car-detailing service and have them wash my car. They'll spend four hours on it. They do everything I do but much more thoroughly. They'll also wax the car, shine the dashboard, and so on. I watched one time, and they used tiny cotton swabs to clean out the little places too small to reach with a rag. That's a lot of effort for slightly better results. For me, the law of diminishing returns kicks in well before I'll use a cotton swab on my car. We want to remain aware, too, of the diminishing return on time spent estimating. We can often spend a little time thinking about an estimate and come up with a number that is nearly as good as if we had spent a lot of time thinking about it. The relationship between estimate accuracy and effort is shown in Figure 6.1. The curve in this graph is placed according to my experience, corroborated in discussions with others. It is not based on empirical measurement.