Their computer then uses statistical formulas to calculate the ratings of each participant, based on their performance in the tournament.
Ratings can be provisional or established .
Until you have completed 20 tournament games, your rating is called provisional, and is not considered completely reliable. After 20 games, your rating is established, and considered fairly reliable. Established ratings still change with each tournament, but are unlikely to fluctuate more than 100 points or so unless your playing ability changes significantly. Ratings can be used to predict future performance. If one player is rated 100 points more than another, he will probably win about 6 games out of every 10 they play.