When people buy shares in an Index Fund, they assume that they are buying shares in the largest companies available in the United States. You look at Apple, see its $700 billion market capitalization, compare it to the 4% allocation in the S&P 500, and figure it sounds about right. And for much of the 1950s, 1960s, 1970s, 1980s, 1990s, and 2000s, this was true. But in 2005, the S&P 500 shifted from selecting stocks based on market capitalization to selecting stocks based on a market capitalization with a formula that takes into account the free float of the stock.
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