In other words, if the S&P has appreciated 8% a year over the last 100 years what percentage of that gain was made on the open? Any experienced trader have a good guess? This is a very important statistic since if a reasonable chunk of the market's gains are made intraday and you pay no margin interest on itraday trades you can make an expected return simply by buying the SPY on the open and selling on the close every day. This is assuming your bankroll is large enough to cover the commisions. This is also a statistic everyone who shorts should know. I've personally never shorted a start since the clock is working against you. e.g. if we assume 50% of the market's gains are intraday than half of 8% divided by 260 trading days means you would typically lose .015% each day if you shorted the S&P randomly.