SPY is an approximation of the S&P index.Quote from fluttrader:
When I go to yahoo and compare charts of the 2, they don't match ?!
http://finance.yahoo.com/q/bc?t=1d&s=^GSPC&l=on&z=l&q=l&c=spy
It could never be exact for two main reasons:
1. The S&P has many decimal places in their composing stocks proportions, so it would require billions, or even trillions to get have the stock in exact proportions.
2. The index is always changing (by some small amounts), because the index is capitalization weighted, if some stock increases in price, its proportion also increases.
