In calculating an index, e.g. the retail price index or the Financial Times index, the various figures are related to a chosen base date. The value of the index at any particular time is expressed as a percentage rise over, or decrease from, the value at the date chosen as the base, usually represented as 1(X), e.g. if an index of prices is stated to be 164 then that figure is only meaningful if related to the date on which it is based and should properly be stated as. say, 164 (base 6 January 1962) or 164 (6 January 1962 =100).
|Reference: The Penguin Business Dictionary, 3rd edt.|