The fund seeks to track the performance of a market-weighted Treasury index with an ultra-short-term dollar-weighted average maturity. The fund employs an indexing investment approach designed to track the Bloomberg Short Treasury Index (the Index). The Index includes fixed income securities issued by the U.S. Treasury (not including inflation-protected bonds, floating rate securities and certain other security types), that have maturities up to 12 months. The fund invests by sampling the Index, meaning that it holds a range of securities that, in the aggregate, approximates the full Index in terms of key risk factors and other characteristics. Under normal circumstances, the fund will generally invest all, but at least 80% of its assets in the securities comprising the Index and in securities that the advisor determines have economic characteristics that are substantially identical to the economic characteristics of the securities that comprise the Index. The fund seeks to maintain a dollar-weighted average maturity consistent with that of the Index, which, under normal circumstances, is expected to be between 1 and 12 months.