productivity

here is an interesting link concerning productivity in software projects. The model is called the devil's quadrat (by Harry Sneed) and visualizes the interdependences between the following 4 characteristics of a typical project:
quality,
quantity,
time,
and cost.
Productivity can be measured in either of these; however, ignoring the mutual dependencies between the factors can have tremendous effects on your estimates.

The reason is that a team's productivity is invariant; it can not be changed (easily). In the model shown on the web page it becomes clear that each one of a projects characteristics can be changed only at the expense of the others. For example, if one wishes to have high quality, the project will unavoidably take longer to complete, be more expensive, and provide a system with less functionality.

The other side of applying the model is to see how far one can get under given constraints. If, for example, your budget and time are set already (as is the case in many projects) you can (starting from a model of ideal circumstances) estimate how much you can achieve with the resources at hand.