What is a result-effective variable in patent law?

This page is an FAQ based on guidance from the Manual of Patent Examining Procedure. It is provided as guidance, with links to the ground truth sources. This is information only: it is not legal advice.

A result-effective variable is a parameter that is recognized in the prior art as affecting a particular result or outcome. The concept is important in patent law, particularly in obviousness determinations. According to MPEP 2144.05:

“A recognition in the prior art that a property is affected by the variable is sufficient to find the variable result-effective.”

The idea behind the result-effective variable analysis is that a person of ordinary skill would be motivated to optimize a parameter if there is evidence in the prior art that the parameter affects the result. However, it’s important to note that after the KSR decision:

“[T]he presence of a known result-effective variable would be one, but not the only, motivation for a person of ordinary skill in the art to experiment to reach another workable product or process.”

Applicants can rebut a prima facie case of obviousness by showing that the claimed variable was not recognized as result-effective in the prior art, but this is considered a narrow exception.