Most of the broadly-used market indexes today are "cap-weighted" indexes, such as the S&P 500, Nasdaq, Wilshire, Hang-Seng and EAFE indexes. In a cap-weighted index, large price moves in the largest components can have a dramatic effect on the value of the index. Some investors feel that this overweighting toward the larger companies gives a distorted view of the market, but the fact that the largest companies also have the largest shareholder bases makes the case for having the higher relevancy in the index.
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