Pascal,

Although it might prove interesting to include evolving measure(s) of volatility to improvement the Market timing model, my intuition tells me that it won't make a great difference. Absolute non volatility measures of OB/OS do a very good job of estimating value (RSI & STO are absolute measures).

In my humble opinion, the Model needs to be confronted with a new independent variable in what could be a multiple factor model. Again, one of your friend's Billy indicator comes to mind: MA[$TICK] (600 minutes) slope would do a good job of measuring "the trend" and it is independent because it has nothing to do with volume analysis.

But the point here is not the variable to pick given that you are in a better position that any of us to figure out the best one(s) to select, but the addition of other variables.

Pierre Brodeur