Developed by the EPA's Risk Assessment Forum, this report and accompanying FAQ document were released in July 2014. They respond to recommendations that the agency should incorporate probabilistic analysis into its decision making.
Probabilistic Risk Assessment (PRA) "is a group of techniques that incorporate variability and uncertainty into the risk assessment process," according to the Risk Assessment Forum webpage featuring the release, available here. The original draft white paper was released for public comment in 2009, followed by external peer review in 2010. The Forum site provides links to the original paper, submitted comments, the peer review report, and EPA's responses, as well as the recently released white paper.