Quote from ulla:
Well I was using the multiple bands to distinguish noise from trending, because each successive band should contain more of the price action. So once price action reaches say the 3rd SD, then we should have a higher probability of seeing heavy trending or a snap back to the mean.
Look at your chart from 1/19 through 1/21. Price was outside the 3rd band for a while. Could you have survived that big of a draw down?
BB mean reversion systems work awesomely until price trends too hard one day and you blow your account. It may happen on the first day, or it may happen 4 years from now.