The Density Ratio Class is a collection or family of probability distributions.
Definition
For continuous random variables, the Density Ratio Class is defined as the set of probability density functions (PDFs),
where and are two bounded non-negative functions (non-normalized densities) such that for all in the domain of the random variable and .
\backslash\mbox{PDFs\ } f : \exists c : l(\theta) \le cf(\theta) \le u(\theta) \sim\ \forall \theta