To pursue its goal, the fund generally invests in securities that are included in the ICE AMT-Free Core U.S. National Municipal Index. The index measures the performance of U.S. dollar denominated investment grade tax-exempt debt publicly issued in the U.S. domestic market by U.S. states and territories as well as their political subdivisions. To be eligible for inclusion in the index, securities must be exempt from federal taxes and must not be subject to alternative minimum tax (AMT). In addition, securities must have at least a one-day remaining term to final maturity, a fixed coupon schedule (including zero coupon bonds) and an investment grade rating (based on the middle rating of Moodys, S&P and Fitch). If rated by all three agencies, two of the three ratings must be Baa3/ BBB minus or higher; if rated by two agencies, the lowest rating must be Baa3/ BBB minus or higher; and if rated by a single agency the security must be rated Baa3/ BBB minus or higher. Securities must have at least dollar 25 million currently outstanding face value and must be part of a deal with an original offering size of at least dollar 100 million. The index excludes certain types of securities, including, among others, step coupon securities, taxable municipal securities, floating rate notes and variable rate demand obligations or notes and private placements. The index is market capitalization weighted and.