The "Index" unit of measure relates the value of a variable (or group of variables) to a base level. The base level is set so that the index produces results that are easy to understand and compare. The base level will always be the first period you selected.
The formula for calculating an index is as follows:
x1=(dv1/dv1)*100 = 100
x1 = index of first year selected, x2 = index of second year ...
dv1 = datavalue for first year selected, dv2 = datavalue for second year ...