The favorite longshot bias in tennis tournaments

The first referring to the favorite longshot bias was (Griffith, 1949). Griffith was a psychologist who studied the US horse races. He looked at the ability of people to correctly interpret the winning chances of horses in several races. Griffith found that bettors were very good at forecasting the probable winners but were not good at dealing with extremes: the bettors underestimated the chance of winning for extreme favorites and overestimated the chance of winning for extreme non-favorites. After this publication numerous others researched this bias. The early literature focused on race track settings, like horse, dog and boat races. Subsequently people looked for the favorite longshot bias in other sports. For example soccer (Cain, Law, & Peel, 2000), American football (Zuber, Gandar, & Bowers, 1985) and baseball (Woodland & Woodland, 1994). The favorite longshot bias is found in many countries. The first papers examining this bias did their research in the US (Ali, 1977) or UK (Dowie, 1976) and found the bias in both countries. After these early papers people investigated several other countries and found the favorite longshot bias in nations far apart from each other. For example from Germany (Winter & Kukuk, 2006) to New Zealand (Gandar, Zuber, & Johnson, 2001). Literature examined the existence of the favorite longshot bias in all kind of sport betting mechanisms. The three most used sport betting mechanisms are pari-mutual betting, fixed odds betting and point spread betting. Studying these three mechanisms seemed to give different outcomes with respect to the favorite longshot bias. For example, the point spread betting mechanism did not display the favorite longshot bias. While the pari-mutual mechanism did generate a favorite longshot bias most of the times. So not all literature demonstrated the favorite longshot bias. For example (Woodland & Woodland, 1994) found a reverse longshot bias in the US major league baseball and the national hockey league. Also research in Asia gave no evidence for a favorite longshot bias, according to (Coleman, 2004). In this paper research is focused on the tennis sport where available literature is really scarce. One of the few papers that did some research on tennis is (Cain, Law, & Peel, 2003), who researched 91 tennis matches on grass at the Wimbledon championship. They did not find a favorite longshot bias. Another paper investigating tennis was written by (Forrest & McHale, 2007). They examined tennis matches during three seasons at the highest level (ATP) and found the favorite longshot bias. The aim of this paper is to investigate if the favorite longshot bias occurs in the tennis betting market in the period 2010, 2011 and 2012. Special attention is paid to Grand Slam tennis tournaments. If the favorite longshot bias indeed exists in the tennis betting market, according to (Williams & Paton, 1998), a favorite longshot bias would disappear when looking at high profile tournaments in tennis such as Grand Slam tournaments. Due to the size of these tournaments every contestant will be at the top of his game and therefore insider’s information is limited. A second feature that is taken into account is the surface tennis is played on. Thirdly, the feature of home advantage is examined. For a tennis player this means playing in the country of his or her nationality. Other literature only discussed home advantage as a team playing at their own club. For example (Schnytzer & Weinberg, 2008) found a favorite longshot bias when focusing on home advantage. An added value of this thesis with respect to other literature is the high number of matches played in tennis between huge underdogs and heavy favorites that was taken into account. This is because of the dominance of a small number of tennis players in the world in the period 2010-2012. This results in more matches played in the highest and lowest category (these categories have the main focus). As a result there are smaller intervals per category possible, for example in (Forrest & McHale, 2007) each prior match winning probability category has an interval of 10%. This thesis exhibits intervals of 5%. Another advantage of studying tennis is that tennis is an individual sport. Therefore the betting on players is not biased by a huge fan base as in team sports. Normally this huge fan base would consequently bet on the team they are supporting, even if their team is the heavy underdog, thereby highly influencing the odds. In this thesis the tennis matches played in the years 2010, 2011 and 2012 that were available to be betted on at Bet365 were examined. The matches investigated were played at the highest level possible in tennis, namely the ATP level. This thesis found for the period 2010-2012 a favorite longshot bias in the tennis betting market. This favorite longshot bias also occurred for each year separately. Looking at the home advantage there were no differences for the whole period. If only home games were considered, a favorite longshot bias still occurred, but the overall return was three times less negative as for the whole sample. The subsample of four different surfaces, namely clay, grass, indoor hard-court and outdoor hard-court, showed a favorite longshot bias for the surfaces clay, outdoor hard-court and indoor hard-court. Only grass gave a different outcome: in contrast to the other surfaces, the underdog category gave a positive return on grass. So for the other surfaces the underdogs were over betted, while on grass the underdogs were under betted.