IWB tracks the Russell 1000 Index, which holds the 1,000 largest US companies by market capitalization. The index covers most of the market value of US stocks and is cap-weighted, so the largest technology and growth companies account for a large share of returns. Compared with the S&P 500, the Russell 1000 includes about twice as many companies, including a number of mid-sized ones, and is reconstituted annually using rules rather than a committee. Over long periods IWB has performed very similarly to S&P 500 funds.
