The risk-return ratio is a measure of return in terms of risk for a specific time period.
simply refer to the price by the start and end of the time period.
The risk is measured as the percentage maximum drawdown (MDD) for the specific period: where DDt, DDt-1, Pt and Pt-1 refer the drawdown (DD) and prices (P) at a specific point in time, t, or the time right before that, t-1.
However, the RRR can arguably be regarded as more general than the MER ratio since it can be used for any time interval even daily or intra-day prices, while the MER ratio seems to be confined to measuring only the risk and return of a fund since inception until the current date.
The RRR was first defined and popularized by Dr. Richard CB Johnsson in his investment newsletter ('A Simple Risk-Return-Ratio', July 25, 2010).