Measurements of a software project is collected (e.g., effort in man-years, elapsed time, and lines of code) and an equation fitted to the data using regression analysis.
SLIM (Software LIfecycle Management) is the name given by Putnam to the proprietary suite of tools his company QSM, Inc. developed, based on his model.
While managing R&D projects for the Army and later at GE, Putnam noticed software staffing profiles followed the Rayleigh distribution.
The points along the curve represent the estimated total effort to complete the project at some time.
One of the distinguishing features of the Putnam model is that total effort decreases as the time to complete the project is extended.