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Evidence for the Favourite–Longshot Bias in Tennis Betting Odds

Tennis match betting, with only two possible results in a game, offers potentially better value than football fixed odds, which also has the draw. With the exception, perhaps, of the most popular football leagues like the English Premiership and Spanish La Liga, overrounds for tennis match betting will generally be lower. Indeed from a sample of 7,957 matches played between the 19th April 2010 and the 15th May 2012 and analysed for evidence of a favourite–longshot bias, the mean overround from average betting prices collected by the odds comparison website oddsportal.com was just 106.4%, whilst for best prices it was 100.0%, the equivalent of fair odds. The results of the analysis are tabulated below.

Maximum betting price	Number of bets	Return
Odds ≤ 1.25	2,260	100.86%
1.25 < odds ≤ 1.5	2,579	103.05%
1.5 < odds ≤ 2	3,150	100.96%
2 < odds ≤ 3	3,210	99.97%
3 < odds ≤ 5	2,593	92.76%
Odds > 5	2,122	81.10%
All odds	15,914	97.10%

As for fixed odds European league football, ATP and WTA tennis exhibits a pronounced favourite–longshot bias. Betting any average price at evens or shorter loses the punter less than 4 pence in the pound. By contrast betting anything longer loses him nearly one fifth of his investment, That rises to nearly a half for bets over 5.00; a staggering outcome. Inefficiency in the betting market persists even at best available prices, although like for football it is weak and exists only really for the longer prices beyond 3.00.

Incredibly, had a punter backed every favourite at the best available price over this 2-year period, his return on investment would have been nearly 102% from almost 8,000 matches. At £100 per bet that translates into almost £13,000 profit in a little over 2 years, simply from using an odds comparison, considerably better than many who claim to be "sports tipsters". One could not, of course, rely on such profit being replicated time and time again. Perhaps more importantly, however, many of the bookmakers who offer such arithmetic value prices in this betting market have reputations for expressing a dislike to punters found to be taking advantage of them. I'll take a more in-depth look at bookmakers' attitude to risk next time.

The overriding conclusion to be drawn from all of this is that if you don't know your tennis (and football for that matter), don't ever back the underdog; you're certain to lose a higher proportion of your turnover.

Next time: Most Bookmakers don't like Winners.