Définition
The index of a number is the ratio between the value of the number in the course of a current period and its value in a baseline period. It measures the relative variation of the value between the baseline period and the current period. Often, the ratio is multiplied by 100 and we say "index base 100 at such-and-such a period".
Indices are used for easy calculation and comparison of the trends in several numbers between two given periods.