The "perpetual inventory method" (PIM) used to estimate fixed capital stocks was invented by Raymond W. Goldsmith in 1951 and subsequently used around the world. The basic idea of the PIM method is, that one starts off from a benchmark asset figure, and adds on the net additions to fixed assets year by year (using gross fixed capital formation data), while deducting annual estimates of economic depreciation based upon an explicit service life assumption, all data being adjusted for price inflation using a capital expenditure price index. In this way, one obtains a time series of annual fixed capital stocks. This data series can also be modified further with various other adjustments for prices, economic depreciation, etc. (several variants of the PIM approach are nowadays used by economic historians and statisticians).