5. The method of claim 4 , wherein the respective plurality of sample sizes is determined by: determining a respective plurality of sample probabilities associated with the respective one of the plurality of query keyword subsets; determining the respective plurality of sample sizes according to a predetermined query keyword sample size and the respective plurality of sample probabilities.
6. The method of claim 5 , wherein determining the respective plurality of sample probabilities includes: determining a sum of PV values (SPV value) of the query keyword subset; determining a total number of PV values in the plurality of query keyword subsets according to the SPV value; determining a sample probability of the query keyword subset according to the SPV value and the TPV value.