Comeback Factor (CBF): The measure of how big a comeback is in a game. CBF is defined as the inverse of the winning team’s lowest Win Probability (WP) during a game. For example, it a winning team’s lowest point in a game is 0.10 WP, its CBF would be 10, that is 1 / 0.10. The higher the CBF, the bigger the comeback.
Excitement Index (EI) – The measure of how exciting a game is. EI measures the total movement of the Win Probability (WP) line during a game. The more that WP fluctuates, the more dramatic, uncertain, and exciting a game is.
Win Probability Added (WPA) – The difference between a team’s Win Probability (WP) at the start of a play and the WP at the end of the play. WPA is the measure of a play’s impact on the outcome of a game. An individual player’s WPA is the sum of the WPA of the plays in which that player was directly involved. Being directly involved is defined as an offensive player who ran, threw, or kicked the ball, was targeted by a pass, or flagged for a penalty. Defensive players are credited for WPA when they tackle or sack the ball carrier, are credited with an assisted tackle or sack, cause a fumble, defend a pass, or are flagged for a penalty.
Expected Points Added (EPA) – The difference between the Expected Points (EP) at the start of a play and the EP at the end of they play. EPA is the measure of a play’s impact on the score of the game. An individual player’s EPA is the sum of the EPA of the plays in which that player was directly involved. Being directly involved is defined as an offensive player who ran, threw, or kicked the ball, was targeted by a pass, or flagged for a penalty.
Success Rate (SR) – The proportion of plays in which a player was directly involved that would typically be considered successful. Specifically, SR is the percentage of plays resulting in positive Expected Points Added (EPA).
Positive Win Probability Added (+WPA) – The Win Probability Added attributed to a defensive player, limited to only the plays that are net positives for his team. It is a measure of a defender’s impact on the outcome of games in terms of play-making ability. Only positive plays are considered for individual defenders because very good individual plays can still result in net losses in WPA. For example, a safety who makes a shoe-string tackle to stop a TD would be a great individual play, but the play as a whole would still be a net negative outcome for his team. However, overall individual WPA likely correlates well with +WPA.
Positive Expected Points Added (+EPA) - The Expected Points Added attributed to a defensive player, limited to only the plays that are net positives for his team. It is a measure of a defender’s impact on the game score in terms of play-making ability. Like for +WPA, only positive plays are considered for individual defenders because very good individual defensive plays can still result in net losses in EPA. However, overall individual EPA likely correlates well with +EPA.
Tackle Factor (TF) – The ratio of a player’s proportion of his team’s tackles compared to what is expected at his position. For example, middle linebackers in a 4-3 typically make 11.9% of their team's tackles. A MLB who made 12.6% of his team's tackles would have a TF of 11.9/12.6 = 1.06. TF is adjusted for a full 16-game season.