70pts is around 1 standard deviation of the daily returns on the dow - 50pts is around 1 stdev of the distribution of intraday highs and lows from the opening price (using 15min bar data) - in genereal, i guess that perhaps for these reasons, plus the "round figure" factor, these levels might be inputs into statistical models - i ran the numbers a couple of months ago, so the above figures are from memory.