The method for building the index was based on a technique developed by James Estey in his book Business Cycles published in 1950. Estey included a chart of business cycles from 1790 through 1949 in which he plotted economic activity similar to GDP. Armed with dates for business cycle peaks and troughs, he de-trended the data by calculating the average for the series between peaks, then between troughs, then averaging the two series to obtain an average for each business cycle. Then he subtracted monthly figures from the cycle averages.
