Dont forget about the trust ratings and the posts quality of the account, using only post count and activity will give result which is very far from being correct
I think...
1. Post quality can not be determined mathematically. It is a subjective concept. Word count may return totally wrong figure as many bots are now posting garbage and copy-pasting long post by other users.
2. Yes. Trust matters. But, only +ve & -ve. From the perspective of signature campaign, a +ve trust account is not much valuable than a neutral trust account, given all other parameters are same.