What determines the rating of expert on the PG?

actually the question in the topic title! thought from the date of registration or the number of comments and all kinds of activity on the site, but it's different for everyone who zaregan in the 14th and a rating of 2 someone 8 and a rating of 7 and it is not clear it is better higher or lower figure? how to improve the result! yesterday I had a rating of 7, and 6 today do not know good or bad,