Logit models relating whether a drug has post-approval independent secondary patent claims to sales and approval year.

Notes:
*indicates p<.05;
**p<.01;
***p<.001; Standard errors in parentheses. Based on the 342 new molecular entities (with at least one patent) approved by the U.S. Food and Drug Administration between 1991 and 2005. Categories are based on authors' coding. “PIPES” refers to Polymorph, Isomer, Prodrug, Ester, and Salt claims. Independent patents are those with no chemical compound claims. Sales categories are based on national estimates of sales (using information from the Medical Expenditure Panel Survey 1996–2010) in the fifth year after brand drug approval. The left-out category is the bottom sales quartile.