Revenue Procedure 2013-27 provides guidance with respect to the United States and area median gross income figures that are to be used by issuers of qualified mortgage bonds, as defined in § 143(a) of the Internal Revenue Code, and issuers of mortgage credit certificates, as defined in § 25(c), in computing the housing cost/income ratio described in § 143(f)(5).
http://www.irs.gov/pub/irs-drop/rp-13-27.pdf