Laspeyres index

A quantity or price index for an aggregate in which the formula weights are taken from a designated base period and do not change over time. For example, in computing a Laspeyres quantity index, the quantities in each time period are multiplied by the base-period prices. Laspeyres indexes are relatively easy to compute and analyze, but they do not capture substitutions between items over time, and their movements are highly dependent on the choice of the base period.