Poker standard deviation, with a twist

[elitegimp]

So for those of us that are madly in love with Pokertracker, we are used to thinking about our standard deviation in terms of BB/100. As far as I know, poker tracker groups your hands in 100s (i.e. hands 1-100, 101-200, 201-300, ...), then does the typical SD calculation of summing up the square of the difference from the mean and dividing by n or (n-1) or whatever. Is there a way to calculate the standard deviation _per session_ rather than _per 100 hands_? Would such a statistic even make sense?

I'm thinking in terms of some sort of weighting, but don't really know how to do it. Here's an example, though, of 400 hands played over 3 sessions:

Session 1 (160 hands):
100 hands, +2 BB
60 hands, -1 BB

Session 2 (180 hands):
40 hands, +3 BB
100 hands, +2 BB
40 hands, +5 BB

Session 3 (60 hands):
60 hands, -3 BB

So my understanding (and this may be wrong) is that poker tracker sorts the hands in this manner:

100 hands: +2 BB
100 hands: +2 BB
100 hands: +2 BB
100 hands: +2 BB

and obviously the SD is 0 BB/100 hands (even if I don't know if it is 0*(1/3) or 0*(1/4) [img]/images/graemlins/smile.gif[/img]).

But if we look at the data as
160 hands: +1 BB
180 hands: +10 BB
60 hands: -3 BB

and use a mean of 0.02 BB/hand is there a way to get a reasonable standard deviation? I'm thinking along the following steps --

1) Make an Expected Value chart based on 0.02 BB/hand, like such:
160 hands: +3.2 BB
180 hands: +3.6 BB
60 hands: +1.2 BB

2) For each session, find the square of the deviation from the expected value
160 hands: (2.2 BB)^2 [EV of 3.2 BB, actual of 1 BB]
180 hands: (6.4 BB)^2 [EV of 3.6 BB, actual of 10 BB]
60 hands: (4.2 BB)^2 [EV of 1.2 BB, actual of -3 BB]

3) So obviously there is some variation from session to session, but how do you weight it? And how do you interpret the results?

It seems clear to me that the SD should be between 2.2 BB/session and 6.4 BB/session, but beyond that I'm pretty lost.

Did anybody make it this far with even a vague notion of what I'm trying to do? Anyone have any thoughts?