For a continuous random variable, the ratio, R(x), of the survivor function (see hazard rate) to the probability density function:
Investigations of the ratio have mostly been undertaken with respect to the standard normal distribution, for which, for x>2, ![Mills ratio](Images/oree/doc/10.1093/acref/9780199679188.001.0001/acref-9780199679188-math-0416-full.gif)