Vanguard Extended Market ETF(AMEX:VXF)

The fund tracks an underlying index that is constructed by removing the S&P 500 companies from the S&P Total Market Index, which is a broad index that's more inclusive than the S&P Composite 1500 Index. The resulting portfolio consists of everything from mid-caps to micro-caps. As such, the portfolio is meant to complement an existing allocation that relies on the S&P 500 for large-cap exposure. Matching the fund with another large-cap index will likely result in overlaps and omissions. Overall, the fund provides comprehensive, vanilla exposure to the segment. The fund uses a sampling strategy to track its index, which is rebalanced on a quarterly basis.