The software equation proposed by Putnam, relates the size of software, effort and time taken in a non-trivial way. The result is defined as the Process Productivity. It is mentioned that a true measure of productivity should be Process Productivity and not the conventional measure of productivity defined as KLOC/(Person-week). This conventional measure has been criticized because its value varies widely at any size the estimator is considering and even more widely from one size to another.
The Putnam equation [Putnam L.H., and Myers W., Five Core Metrics – The Intelligence Behind Successful Project Management, Dorset House Publishing, New York, 2003.] as it is known, explains a relationship between software size, effort spent and time taken to deliver the software. It relates these quantities with what is termed process productivity. The equation in words is
Size (at Defect Rate) = Effort x Time x Process Productivity (1)
The equation in its generic form is
Size (at Defect Rate) = Efforta x Timeb x Process Productivity (2)
After studying various projects, Putnam found the relationship as
Size (at Defect Rate) = (Effort/β)1/3 x (Time)4/3 x Process Productivity (3)
This is called the software equation. Beta (β) is a size dependent parameter that has the effect of giving greater weight to the effort factor in very small systems).
It is a real surprise that Software development organizations havent used this as widely as they could. In fact the evidence of experimenting with this coupled metric is also not there. May be we need to explore this more!