A rating system is an attempt to quantitatively represent the strengths of individuals engaging in a competitive activity. Perhaps the best known rating system is the so-called Elo system, developed by Arpad Elo for rating chess players. The Elo system is related to methods used in psychology and other areas for paired preference comparisons. Similar systems have been used with other games, such as go, and for competitive sports such as table tennis. These systems produce a rating for each participant, and include a model (or formula) which gives the probability for game outcomes, based on the difference between the two contestants' ratings. Other rating systems are possible, for example, systems which attempt to model match scores rather than match outcomes.
The accuracy of the ratings depend on getting accurate match results, including indicators for matches played on neutral sites, as well as on the number of games played. In the rating system I am using, the ratings are not an absolute (or ratio) scale; only the differences between ratings are informative. Ratings can't be compared across different rating pools. For example, the ratings of NCAA women's teams are not comparable to ratings of NCAA men's teams, although the distribution of ratings within each group is informative about the range of strengths within the rating pools.