In economics an index takes 2 numbers and expresses the change between the 2 numbers in percentage terms.
An index takes original data e.g. basket price of goods in an economy and expresses this original data as the base year. (This is equal to 100.) The new price for the Basket is then used to calculate a percentage change. Take the New basket price, subtract the Base Year price and divide this number by the Base year price . This number is then multiplied by 100 to get a percentage change.