This column is the third in a series about estimating. This month, we discuss size estimating. We also introduce the subject of proxies and describe how they can help you to make better estimates. The first column in July gave an overview of estimating and the August column talked about software size. If you have not read those columns, you should probably look at them first to understand the context for this discussion and to see how the various estimating topics in these columns relate. To repeat what I said in the last two columns, the estimating method described here is called PROBE. If you want to quickly learn more about PROBE, you should read my book A Discipline for Software Engineering, </A> from Addison Wesley. This book introduces the Personal Software Process (PSP)SM, which is an orderly and defined way for software engineers to do their work.
This column starts the discussion of how to make size estimates. As we discussed last month, to make a project plan, you need an estimate of development resources. To estimate resources, however, you generally need to estimate the size of the product you plan to build. The reason to start with an estimate of product size is that large products generally take more time to develop than small ones. Thus, when you have good historical productivity data, a good size estimate will provide a sound basis for estimating development hours.