01/01/1993 - 31/12/2013 periods...using a daily chart.
there were 5,529 real trading days.....all weekends and public holidays have been stripped out.
ALL relevant time adjustments for USA summer time have been accounted for too etc etc.
When the SP500 opening bell opens UP and the market subsequently closes down it occurred 948 times a 17.15% occurrence. Almost a 1 in 5 chance of this occurring if you like.
BUT when you look at recent data say just the 2014 period we have 252 trading days whereby there were 30 such outcomes or if you prefer a 11.9% occurrence ....as a comparison that is almost a 1 in 8 chance of this occurring.
So if sentiment remains as positive as we have seen it may be wiser to follow the momentum.