 Unlike the "reward-to-variability" ratios, such as Sharpe ratio and Sortino ratio, the Rachev ratio is a "reward-to-risk" ratio, which is designed to measure the right tail reward potential relative to the left tail risk in a non-Gaussian setting.Intuitively, it represents the potential for extreme positive returns compared to the risk of extreme losses (negative returns), at a rarity frequency q (quantile level) defined by the user.The ratio is defined as the Expected Tail Return (ETR) in the best q% cases divided by the Expected tail loss (ETL) in the worst q% cases.