Indirect patent citations

Patentcitations are extensively used as a measure of patent quality. However,counting citations does not account for the fact that citations come frompatents of different qualities, and that citations are of variable qualities.We develop a citation index which takes into account the cumulative quality ofthe citing patents. We apply this index to the 2,139,314 utility patentsgranted in the U.S. between 1975 and 1999. We study the properties of thisindex by year and by technological category, and analyse the links betweenpatents.