Another study done on SPY (the tradable SPX ETF) showed buying the opening price and selling the closing price from 1993 to April 2010 would have produced a loss of -38%.
But buying the closing price and selling the opening price the next day would have produced a gain of +378% in the same time period.
This does confirm the old adage being " The open is determined by amateurs. The high determined by the bulls. The low determined by the bears. And the close determined by the professionals."