Economic, or cyclical, indicators signal changes in the direction of aggregate, economic activity. Defined and analyzed by the National Bureau of Economic Research (NBER) since 1938, there are three types of cyclical indicators: leading, coincident, and lagging. The list of indicators is periodically revised. Unless otherwise stated, all indexes use 1967 as their base of 100 and all dollar-valued series are in 1972 dollars. A complete listing and description of the individual time series and composites can be found in the Handbook of Cyclical Indicators or in the monthly listings prepared by the Council of Economic Advisors and the BEA's Business Condition Digest.