Estimating

One key ingredient that we conveniently skipped so far is the thorny issue of estimation. How big is this project? How many people are needed?

By far the best tool a project manager can use to estimate a project, a phase, an iteration, or a simple activity is historical data. The most basic approach is to record effort, duration, or size estimates as well as estimate processes and assumptions, and then record the actual results from each estimated activity. Comparing actual outcomes to the estimates helps generate more accurate estimates in the future. Estimating procedures and templates that itemize tasks help avoid the common problem of overlooking necessary work. The RUP constitutes such an inventory of activities.

There are estimation tools, such as COCOMO, but they will not tell you how much effort is required for your new project; they will only help you estimate how long and how many people, by adjusting the numbers for various cost drivers, integrating the performance of your team, and assessing the difficulty of the project. For the project plan, early in Inception, it is best to calibrate the new project relative to another one for which you already know the total effort. Then a model such as COCOMO will derive an estimation of duration and level of staffing. Remember that software developers are notoriously nonfungible; this is what makes planning software projects a bit more tricky.

Once the project is started, you might want to combine top-down and bottom-up approaches and exploit not only your growing knowledge of what you want to accomplish, but also that of your development staff. Most likely they have started doing some early prototyping and using their understanding of the tools, from which they can draw better estimates. See the next section for a simple but effective approach: using the best of your people and whatever known input you have.

More sophisticated methods include using the function-points method and its various derivatives. Use cases can be used as a starting point for function-point -like techniques to estimate the "size of the mountain." ^[3]

^[3] See Albrecht 1979.