Index is a variable that is determined by dividing one measurement by another, and it is usually expressed as a percentage. Indices can be used to show variations over time, by comparing subsequent measurements of a variable with an initial measurement (an index of 100 indicates no change). Indices can also show the relative incidence of a particular characteristic in two samples, by dividing the percentage of one sample by the other, indices of 120 and over or 80 and below generally indicate above-average and below-average skews respectively.
