The average size stack, in the lobby, is an overall gauge for how your chip gathering endeavors are progressing.
During a break, when the tournament is no longer in late reg., one can use the total number of entrants stacks ( plus rebuys/addons chips ) and divide by the number of places payed to get a relative average of where you should be to cash.
Table average is good for gauging when to snipe at a stack.
There are numerous ways stacks can be used mathematically during a game, guess that algebra I learned in high school might come in handy.
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