The Standard & Poor’s 500 Index, or simply S&P 500, is a market-capitalization-weighted index of 505 large-cap U.S. stocks. The index accounts for 80% of the market value of the U.S. equities market. Because it reflects nearly all of the largest stocks in the U.S., it is often regarded as synonymous with “the market” as a whole. It’s as close as there is to a “default” U.S. stock index.
Because it’s weighted by market cap, the largest stocks have a big impact on both the long-term performance and daily movement of the index. The 10 biggest stocks make up 27% of the …
