Betting Market Inefficiencies in European Football -- Bookmakers' Mispricing or Pure Chance?

Research on sports betting often identifies biased evaluation by bookmakers and corresponding opportunities for profitable strategies to bettors. Such studies repeatedly provide evidence for the existence of biased betting odds for different periods and leagues, leaving the impression that inefficiencies are very common. Since most studies cover only a few seasons, the question remains whether these market inefficiencies persist over time. We review the literature on the big five leagues in European association football and then analyse 14 seasons to detect the occurrence and duration of betting market inefficiencies. While our results replicate the temporal findings of previous research, they also show that biases do not persist systematically over time and across leagues. Furthermore, a Monte Carlo simulation reveals that the number of inefficient periods barely exceeds what would be expected in an efficient market