The National Basketball Association playoffs start this coming weekend, running until the finals in early June. In recent years, the NBA has been heavily criticized for the quality of its referees, with many fans suggesting that the league is biased towards larger-market, more popular teams, like the Los Angeles Lakers and Boston Celtics. For example, many observers criticized "unfair" refereeing helping the Lakers in 2002 against the Sacramento Kings, and the Miami Heat against the Dallas Mavericks in the 2006 finals. The criticism became even more serious when disgraced former referee Tim Donaghy claimed that referees were in fact fixing game results; Donaghy even suggested specific games, including Game 6 of that Lakers-Kings series. At The Study, though, we prefer our evidence in academic form, so we ask: are pro basketball referees biased?

Yes, say economists from BYU, Johns Hopkins and Oregon State, but not in favor of "bigger" teams. Using play-by-play data from ESPN for regular season and playoff basketball games from 2002-2008, the economists looked at "discretionary" turnovers (traveling violations, offensive fouls, and other turnovers caused by "active ref behavior") and fouls called against both teams, though they caution that the latter measure is "only suggestive, though still important." They found "evidence of three biases: favoritism of home teams, teams losing during games, and teams that are behind in a multi-game playoff series. All three biases are plausibly profit-enhancing for the league." The authors calculate that, during the regular season, the turnover biases "equates to win probability changing by approximately 2.2% when a team switches from away to home status," and a further 2.5% if fouls are included. In the playoffs, the biases do not appear to affect fouls, but the effect on turnovers becomes nearly doubles, keeping the probability change close to 5%. Maybe beating that 5% is why coaches ask players to give 110%...

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