User:Haowen Wan/Win Shares

Win Shares is a revolutionary system that allows for player evaluation across positions, teams and eras, measures the total sum of player contributions in one groundbreaking number. The idea of win share was firstly introduced by  Bill James and Jim Henzler, published by STATS, Inc. in 2002. The book explains how to apply the concept of sabermetrics to assess the impact of player performance in a combination of several areas, including offensive, defensive, and pitching, to the overall performance of their team. A Win Share is a single number that covers the whole shooting match: offense, defense and, if the player in question is a pitcher, pitching. What is especially neat about it is that it takes into account context. A player's home park advantage/disadvantage is factored into the equation as is the era in which he played or is playing This book introduce how author was using the formula to calculate the win share, and how it shows the efficiency of a list of player in the baseball league.

The concept of win shares
Bill James first developed the concept of "win shares" as a way to determine how many victories a baseball player contributed to his team each year in the book, Win share, published by STATS,Inc, in 2020. In the beginning, the concept of win share was only use in the baseball league. In James's system, one win is equivalent to three Win Shares. In general, if the team wins 240 games per season which means the total win share of the player would be 240. Bill James did not allow for the possibility of negative Win Shares. the lowest number of pitcher would be zero which means the the pitcher contribute little to the win a game

In baseball, the contributions of hitting, pitching, defensive contribution takes into account. If a team with almost the same offensive and defensive prowess, hitters receive 48% of the win shares and those win shares are allocated among the hitters based on runs created. Pitching contributions typically receive 35% (or 36%) of the win shares, defensive contributions receive 17% (or 16%) of the win shares. The pitching contributions are allocated among the pitchers based on runs prevented, the pitchers' analogue to runs created. Fielding contributions are allocated among the fielders based on a number of assumptions and a selection of traditional defensive statistics

Disadvantage of win shares

 * In Bill's system, a player cannot receive below zero for his offensive and defensive win share, because Bill think that zero means the pitcher contribute little to win the game. However, it is undenied that allowing the player to get negative scores would be more fair to see the contribution of player on each side. The negative score just means that a player with negative Win Shares was so poor that he essentially took away wins that his teammates had generated.
 * In the initial concept of win shares, Pitching contributions typically receive 35% (or 36%) of the win shares, and hitter receive 48% of the win shares. The unequal contribution of each side would lead an unfair result of win share score.

Who bring win share to NBA
Justin Kubatko of Basketball-Reference.com quickly recognized the value of proportionally dividing credit for team success in basketball, where individual players have a massive influence relative to many other team sports. Kubatko created a new formula using readily available player stats to calculate the number of wins each player contributes to the team each season, and basketball win shares were born.

The reason why NBA use win share
Win share statistics are useful to bettors because they combine multiple indicators into a single number that represents each player’s relative value to the team. These composite data points are extremely useful when considering the impacts of lineup changes following injury or trades.

Combining the win share indicators for the entire team can also provide a helpful second point of reference to judge a team’s overall quality. This is useful as a secondary metric when comparing two teams that have similar win-loss records or otherwise look similar on paper.

The different formula of win share in NBA
In Bill system, a win share means three times the number of win games. If a team wins 70 games, the total win share of player would be 210. However when Kubatko introduce the idea of win share to NBA. Kubatko change the formula of calculating the win share, the general idea of win share representing the points home team earns minus the point home team lose when the player is playing on the court.

offensive win shares
numbers of wins attributes to a player's offensive effort

Defensive win shares
numbers of wins attributes to a players' defensive effort

Overall win shares
Numbers of win attributes to a player's overall effort

Win shares per 48 minutes
Since the total minutes of a game is 48 minutes, thus using ws/48 would be more precise to show the efficiency of a player’s contributions. The formula of win share 48 is use the overall win share to divide by 48. In such, WS/48 is a better metric when handicapping the potential impact of a substitute player coming off the bench or stepping up to take the place of an injured starter. Always remember that a starter’s win shares are inflated by the additional time they spend on the court.

The advantage of using win share
Accounting for personal: In Bills' idea that the win share is the total number of a teams win. However the sum of a team’s win shares doesn’t tell you much about their ability to put points on the board. Strong teams with a slow pace may have lots of players with high win share stats, but that doesn’t mean they’ll create high-scoring games.

Evaluating recently injured players: Injured stars who miss significant playing time will see their win shares take a serious hit, even if they’ll immediately change their team’s fortunes when they return to the court

How to calculate win shares
Kubatko uses separate formulas to calculate win share statistics for three major ‘eras’ in NBA history, making use of the most relevant player stats available for each respective era[2].

The math function of marginal offense
$$(POINTPRODUCTED)-0.92*(LEAGUGEPOINTSPERpossession)*(TEAMDEFFENSEpossession)$$

the math function of marginal points per win.
$$0.32*(leagugepointspergame)*(teampace/leaguepace) $$

The math function of offensive function

$$MARGINALoffense)/marginalpointsperwin $$

the math function of marginal defense
$$(playerminutesplayed/teamminutesplayed)*(teamdefensivepossession)*(1.08*(leaguepointsperpossession)-((defensiverating)/100)) $$

the math function of marginal point per win
==== $$0.32*(leagugepointspergame)*((teampace)/(leaguepace) $$ ==== the math function of defensive win share

$$MARGINALdefense/marginalpointsperwin $$