First Half Hour as a Volatility Indicator

Interesting finding: I went back to 11/29/05 and hourly data in SPY. When the first half-hour in SPY is nearly unchanged (neither rising nor falling more than .05%; N = 33), the median range for the rest of the day is .61%, with only 13 of the 33 occasions hitting a range of .70%. When the first half-hour in SPY rises or falls more than .05% (N = 54), the median range for the remainder of the day has been approximately .75%, with 31 of the 54 occasions reaching .70% or greater. A slow half-hour appears to be a warning sign of low volatility for the remainder of the day. More on this topic shortly.