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Match-fixing in professional sumo
Match-fixing in professional sumo is an allegation that has plagued professional sumo for decades. Due to the amount of money changing hands depending on rank, and prize money, there had been reports of (corruption, bout-fixing) in professional sumo for years before it was finally definitively proven to exist in 2011. The hierarchical structure of the sport, in which a minority of top-ranked wrestlers have great advantages in salary, privileges and status over the lower-ranked wrestlers that make up the majority of sumo participants, may have contributed to the use of match-fixing in order to prolong careers for top-ranked wrestlers and assist in the distribution of promotions.〔Hongo, Jun, "(Sumo-rigging born of necessity? )", ''Japan Times'', February 9, 2011, p. 3.〕〔Kyodo News, "(Ex-sumo attendant says match-fixing nothing new )", ''Japan Times'', February 10, 2011, p. 1.〕
==Previous speculation==
In ''The Joy of Sumo: A Fan's Notes'' (Charles E. Tuttle, 1991), David Benjamin determined that over a span of ten basho in the years 1989 and 1990, wrestlers who entered the final day with records of 7–7 emerged with an unlikely winning ratio of .813 (39–9). He offered further evidence of cheating in sumo, but his analysis of Day 15, "The Last-Day Blues", represents the first effort to demonstrate statistically that rikishi almost certainly trade favors late in sumo tournaments to facilitate the all-important goal of ''kachi-koshi'', having more wins than losses in a tournament.〔Benjamin, David. ''The Joy of Sumo: A Fan's Notes''. New York: Tuttle, 1991. pp. 231–254.〕
In 2002, Steven Levitt and Mark Duggan replicated and expanded upon Benjamin's research, although not crediting ''The Joy of Sumo''. They published a paper using econometrics in order to suggest that corruption in sumo exists. Popularized in Levitt's book ''Freakonomics'', the study found that 70% of wrestlers with 7–7 records on the final day of the tournament (i.e., seven wins and seven losses, and one fight to go) won. The percentage was found to rise the more times the two wrestlers had met, and decrease when the wrestler was due to retire. The study found that the 7–7 wrestlers won around 80% of the time when statistics suggest they had a probability of winning only 48.7% of the time against their opponents. Like Benjamin, the authors concluded that those who already have 8 wins collude with those who are 7–7 and let them win, since the 8-win wrestlers had already secured their ranking.
A possible counter-argument to the ''Freakonomics'' conclusion is that a 7–7 rikishi was highly motivated to win his last match to gain promotion, rather than demotion, while the 8–6 rikishi had already guaranteed his promotion, so was not as motivated. The authors revealed a more damning statistic, however. According to their research, the next tournament in which the two wrestlers met, there was a significant advantage to the 8–6 wrestler over the 7–7, regardless of the performance of either wrestler. The previously 7–7 wrestler would win only 40% percent of the rematches with the 8–6 wrestler. The authors suggested that winning 80% in the first match and then only 40% in the rematch (and back to the expected 50% in subsequent matches) between the same wrestlers suggested a rigging of the bouts. Additionally, the authors found that after allegations of rigging by the media, 7–7 wrestlers won only 50% of their matches against 8–6 wrestlers instead of 80%.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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