she is a market cap index and is re-balanced annually. if apples market cap declines her weighting is reduced. watch for funds that track the nasdaq 100 to sell apple shares to retain order in 2013.
best pair trade of 2013 buy ``qqq`` short ``apple``. funds that track nasdaq 100 must now sell apple and re-allocate index weighting dollars to the companies who have increased in market capitalization.
Stocks rebalance on their own as their stock price changes. For example, AAPL drops 20% in price. That reduces its value as a part of the index by 20%. There is no need to buy or sell stocks to rebalance.