When a match is analysed, each player's actions are recorded. For each of these actions, a player earns or loses points - for example, a goal is worth up to 600 points, whereas a short pass in a player's own half only earns ten points and a foul costs a player 50 points.
Over the course of a game, a player will accumulate a total number of points to give him a Game Score. This can be used to compare his performance with other players and the player with the most points is nominated as Opta's man-of-the-match.
The Index, which has featured in many newspapers and on television, uses these Game Scores to provide a form guide which is calculated over a period encompassing the previous six Premiership matches.
Each player's points from the last six games are added to give him a total, which is then divided by the number of minutes played and then multiplied by 90 (minutes), to give him an average score per game played. This is his Index Score.
These Index Scores can then be used to compare players' performances over those six games, with the player who has the highest average being the most in-form player.
The players are then divided by the position that they play - goalkeepers, defenders, midfielders, attacking midfielders and attackers - to create a series of tables providing an at-a-glance guide to current Premiership player form.
Opta also produce a Season Index. This shows each player's average score across every game they have played during the season.
The Opta Index is basically a form guide.