If history is any guide, the incredible run in Robinhood stock this year probably isn't sustainable.

The S&P 500 ( ^GSPC -0.18% ) is made up of 500 companies from 11 different sectors of the economy. To be considered for a spot in the index, there are multiple criteria, including that companies must have a market capitalization of at least $22.7 billion and must be profitable. But even after ticking those boxes, a special committee ultimately decides which candidates make the cut.

The S&P 500 is rebalanced once per quarter, meaning stocks that no longer meet the criteria are removed in favor of more suitable names. The most recent rebalance occurred on Sept. 22, when casino operator Caesars Entertainment was replaced by Robinhood Markets ( HOOD -0.54% ) , which operat

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