Don't know if anyone came across this study but here it is fwiw. Using the SPY which is the tradable SPX market this study was done from 1993 through April 2010. The results were: Buying the CLOSE price and Selling on the next regular OPEN price throughout this time period of years produced a "gain" of +378%. And doing the opposite and Buying the OPEN price and Selling on the CLOSE price produced a 'loss' of -38% throughout this period. This confirms the theory that amateurs determine the OPEN price and Professionals determine the CLOSE price. Something to keep in mind when buying or selling or holding.