The Vanguard S&P Mid-Cap 400 ETF employs an indexing investment approach designed to track the performance of the S&P MidCap 400 Index. The Index measures the performance of mid-capitalization stocks in the United States. The Fund attempts to replicate the target index by investing all, or substantially all, of its assets in the stocks that make up the Index, holding each stock in approximately the same proportion as its weighting in the Index.