It is the ratio of the upper bound value of the ninth decile divided by the upper bound value of the first decile. It is the ratio of the average equivalised income of the 10% richest of the population to the poorest 10%. The inter-quintile share ratio S80/S20 measures the richest 20% of the population relative to poorest 20%. These measures (along with the P90/P10 ratio) focus on the tails of the income distribution.