A way of expressing, in percentage terms, the change in some variable from a given point in time to another point in time. For example, suppose output increased by 10 percent from an initial year (1987) to a subsequent year (1988). The index for the base year of 1987 in this example would be 100. 0, while the index for 1988 would be 110. 0. Conversely, if output had declined in 1988 by 10 percent, the 1988 index value would be 90. 0.