Data Through June 1961 Are Based On 25 Year Mortgages Prepaid In 12 Years; Data For July 1961-1965 Are Based On 30 Year Mortgages Prepaid In 15 Years. Source: U.S. Department Of Commerce, Business Condition Digest. This NBER data series m13045 appears on the NBER website in Chapter 13 at http://www.nber.org/databases/macrohistory/contents/chapter13.html. NBER Indicator: m13045