Average incremental patent life from independent secondary patents, by type.

Legend: Based on the 432 new molecular entities (with at least one patent) approved by the U.S. Food and Drug Administration between 1985 and 2005. (Overall 276 have a chemical compound patent, and 156 do not.) Categories are based on authors' coding of the claims in the 1304 patents (1261 distinct patents) associated with these drugs. “PIPES” refers to Polymorph, Isomer, Prodrug, Ester, and Salt claims. “Independent” secondary patents in a category are those with no chemical compound claims. For each row, incremental patent life is calculated only for drugs with at least one patent in the category, i.e. the figures represent average incremental patent life from each type of secondary patent conditional on having at least one patent of that type.