 Calculation weights are derived from the dollar values found on the sampling frames and from trade dollar values compiled by the Bureau of the Census for the base year.Movements of price indexes from one month to another usually should be expressed as percent changes, rather than as changes in index points, because the latter are affected by the level of the index in relation to its base period, while the former are not.Each index measures price changes from a reference period defined to equal 100.0.