offense-defense: Offense-Defense method
In comperank: Ranking Methods for Competition Results

Description Usage Arguments Details Value Design of Head-to-Head values Players References Examples

Functions to compute rating and ranking using Offense-Defense method.

rate_od(cr_data, ..., force_nonneg_h2h = TRUE, eps = 0.001, tol = 1e-04,
  max_iterations = 100)

rank_od(cr_data, ..., force_nonneg_h2h = TRUE, eps = 0.001, tol = 1e-04,
  max_iterations = 100, keep_rating = FALSE, ties = c("average", "first",
  "last", "random", "max", "min"), round_digits = 7)

`cr_data`	Competition results in format ready for as_longcr().
`...`	Head-to-Head expression (see h2h_mat()).
`force_nonneg_h2h`	Whether to force nonnegative values in Head-to-Head matrix.
`eps`	Coefficient for total support.
`tol`	Tolerance value for iterative algorithm.
`max_iterations`	Maximum number of iterations for iterative algorithm.
`keep_rating`	Whether to keep rating columns in ranking output.
`ties`	Value for `ties` in `round_rank()`.
`round_digits`	Value for `round_digits` in `round_rank()`.

Offense-Defense (OD) rating is designed for games in which player's goal is to make higher score than opponent(s). To describe competition results Head-to-Head matrix is computed using ... (see h2h_mat() for technical details and section Design of Head-to-Head values for design details). For pairs of players without common games Head-to-Head value is computed to 0 (not NA). Note that values should be non-negative and non-NA. This can be ensured with setting force_nonneg_h2h to TRUE.

For player which can achieve high Head-to-Head value (even against the player with strong defense) it is said that he/she has strong offense which results into high offensive rating. For player which can force their opponents into achieving low Head-to-Head value (even if they have strong offense) it is said that he/she has strong defense which results into low defensive rating.

Offensive and defensive ratings describe different skills of players. In order to fully rate players, OD ratings are computed: offensive ratings divided by defensive. The more OD rating the better player performance.

Algorithm for OD ratings is as follows:

Compute Head-to-Head matrix using ....
Add small value to Head-to-Head matrix to ensure convergence of the iterative algorithm in the next step. If all values are strictly positive then this step is omitted. In other case small value is computed as the smallest non-zero Head-to-Head value multiplied by eps.
Perform iterative fixed point search with the following recurrent formula: def_{k+1} = t(A) %*% inv(A %*% inv(def_{k})) where def_{k} is a vector of defensive ratings at iteration k, A is a perturbed Head-to-Head matrix, inv(x) = 1 / x. Iterative search stops if at least one of two conditions is met:
- sum(abs(def_{k+1} / def_{k} - 1)) < tol.
- Number of iterations exceeds maximum number of iterations max_iterations.
Compute offensive ratings: off = A %*% inv(def).
Compute OD ratings: od = off / def.

Ratings are computed based only on games between players of interest (see Players). However, be careful with OD ratings for players with no games: they will have weak offense (because they "scored" 0 in all games) but strong defense (because all their opponents also "scored" 0 in all common games). Therefore accounting for missing players might be not a very good idea.

rate_od() returns a tibble with the following columns:

player - player identifier.
rating_off - offensive rating of player. Bigger value indicates better player performance.
rating_def - defensive rating of player. Smaller value indicates better player performance.
rating_od - Offense-Defense rating of player. Bigger value indicates better player performance.

rank_od() returns a tibble of the similar structure as rate_od():

player - player identifier.
rating_off, rating_def, rating_od - ratings (if keep_rating = TRUE).
ranking_off, ranking_def, ranking_od - rankings computed with round_rank().

Head-to-Head values in these functions are assumed to follow the property which can be equivalently described in two ways:

In terms of matrix format: the more Head-to-Head value in row i and column j the better player from row i performed than player from column j.
In terms of long format: the more Head-to-Head value the better player1 performed than player2.

This design is chosen because in most competitions the goal is to score more points and not less. Also it allows for more smooth use of h2h_funs from comperes package.

comperank offers a possibility to handle certain set of players. It is done by having player column (in longcr format) as factor with levels specifying all players of interest. In case of factor the result is returned only for players from its levels. Otherwise - for all present players.

Amy N. Langville, Carl D. Meyer (2012) Who’s #1?: The science of rating and ranking.

Philip A. Knight (2008) The Sinkhorn-Knopp algorithm: Convergence and applications.. SIAM Journal of Matrix Analysis, 30(1):261–275, 2008 (For stopping rule of iterative algorithm).

rate_od(ncaa2005, mean(score1))

rank_od(ncaa2005, mean(score1))

rank_od(ncaa2005, mean(score1), keep_rating = TRUE)

# Account for self play
rate_od(ncaa2005, if(player1[1] == player2[1]) 0 else mean(score1))