well you don't need to know that  and it varies pool to pool.
But it is a good question.  So here is an attempt to explain it.


They all count your good shares submitted to them.

They then  devide them by a time limit

and they say a rate.

That  is only an estimate  because you can toss  lots of (7's)  in a give time frame  and look like you have a higher rate then you do.

Of lots of good shares in hour 1 = high hash
followed by no shares in hour 2 = low hash


the reality is your tosses of the dice  per hour are your true hash rate
but pools only count  your good shares per hour  thus it is an estimate.