Irwin --
I think I'm right, please correct me, if not. I haven't checked it out, in the context of our conversation. I believe Bollinger bands use exponential moving average(s) (EMA) to generate their readings and signals. XDEV is coming from a whole different place. If you would care to download Myst's Excel spreadsheets, as a starting point, and build a significantly better performing XDEV variation (and then sharing it, with us all), using Bollinger bands, I believe Myst, others, and I, would be thrilled.
For your consideration.
Robo