The investment seeks investment results that correspond generally to the performance, before the fund's fees and expenses, of an index called the Zacks Mid-Cap Core Index. The fund will invest at least 90% of its total assets in securities that comprise the index and depositary receipts representing securities that comprise the index. It generally invests in all of the securities comprising the index in proportion to their weightings in the index. The index is comprised of 100 securities selected, based on investment and other criteria, from a universe of mid-capitalization securities including MLPs, ADRs and BDCs.