Weighting
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The index has traditionally been market-value weighted; that is, movements in the prices of stocks with higher market capitalizations (the share price times the number of shares outstanding) have a greater effect on the index than companies with smaller market caps.
The index is now float weighted. That is, Standard & Poor's now calculates the market caps relevant to the index using only the number of shares (called "float") available for public trading. This transition was made in two steps, the first on March 18, 2005 and the second on September 16, 2005.
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