# Putnam–Norden–Rayleigh curve

In the study of software project scheduling, the Putnam–Norden–Rayleigh curve, also known as the PNR curve, is an equation specifying the relationship between applied effort and delivery time for a software project. A PNR curve can be used to determine the least cost time for delivery ${\displaystyle t_{o}}$ up to the limit ${\displaystyle t_{min}}$, the absolute minimal amount of time required to complete the project no matter how many human resources are added. The PNR curve was used to derive the Software equation.[1]

## Equation

The following equation shows the relationship of project effort as a function of project delivery time.

${\displaystyle E_{a}=m\left({\frac {{t_{d}}^{4}}{{t_{a}}^{4}}}\right)}$

Where

• ${\displaystyle E_{a}}$=Effort in person months
• ${\displaystyle t_{d}}$=The nominal delivery time for the schedule
• ${\displaystyle t_{a}}$=Actual delivery time desired