This column is the sixth in a series about estimating. The first was in the July 1996 issue. This column describes how to get a program size estimate from an estimate of a program's object LOC. The steps involve something called linear regression. This column describes what linear regression is, how it helps in making size estimates, and how to do the linear regression calculations.
The prior columns in this series gave an overview of estimating and defined some of the steps in making a size estimate. If you have not read these earlier columns, you should look at them first to understand the context for this discussion and to see how these various estimating topics relate. To repeat what I said in the previous 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, 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 continues the discussion of how to make size estimates. To make a project plan, you need a resource estimate and, to estimate resources, you need to estimate the size of the product you plan to build. Also, to make a good size estimate, you need historical data on the sizes of the programs you have previously written. This and the previous columns describe how to gather these data and how to use them to make the size estimate.