Chapter 5
Model-Based Estimation

It’s pretty simple to model when a software development project will be completed. All you need is to know the size of the project and the rate at which progress will be made. If you divide the size by the rate, you get the elapsed time. If you want to know the cost, just multiply the elapsed time by the per time-unit cost. It’s simple, right?

If your size is “1 Project” and your rate of progress is “1 Project per Year” then your estimated duration is “1 Year.” Just translate all your measurements to these units.

Perhaps that’s too simple to be useful for you. The equation is surely true, but modeling the real world size and rate in terms of what you know and can observe about your project is more complicated. You generally want a model that is “as simple as possible, but no simpler.”^[12]

In Chapter 2, ​Comparison-Based Estimation​, we looked at ways to model the duration of software development by comparing it with prior software development. Here we look to model it by comparing attributes of this software development with prior. Whereas you might lump size and rate together when you estimate by comparison, when modeling it’s natural to start with dividing those two fundamental attributes.

Let’s take a look at how we can model the size and rate variables in terms of things we know, or think we know. Then we’ll consider the type of construction of the model as a whole.