Averages and indexes differ from one another in that an index

is the arithmetic average price behavior of a group of stocks at a given point in time.

measures the current price behavior of a group of stocks in relation to a base value set at an earlier point in time.

always moves up before a corresponding average moves up$,$ and always moves down before a corresponding average moves down.

is of value in$-$and$-$of itself, whereas an average must be compared to a historical figure to have any meaning.