I don't think a single type of 'statistic' is very useful to 'grade' forums.
They would depend on what the forum is about as to how that statistic could be interpreted.
Take a discussion type forum, and the higher the number of posts per member the better.
Take a help site where 'hopefully' the user NEVER has to return and you would want a ratio the opposite - as small a average of posts per member, with just the 'helpers' have high post counts.
As an accountant I can tell you, that statistics can be made to tell you anything you want - just don't expect them to help you decide if the result is what you require in a given situation.
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