We present a novel Bayesian network model for assessing referee bias with respect to fouls and penalty kicks awarded. Unlike previous studies, our model takes into consideration explanatory factors which, if ignored, can lead to biased assessments of referee bias. For example, a team may be awarded more penalties simply because it attacks more, not because referees are biased in its favour. Hence, we incorporate causal factors such as possession, time spent in the opposition penalty box, etc. prior to estimating the degree of penalty kicks bias. The model is applied to the 2011-12 English Premier League season. Among our conclusions are that, in contrast to previous studies, being the home team does not in itself result in positive referee bias. Conversely Arsenal, a team of similar popularity and wealth and who finished third, benefited least of all 20 teams from referee bias at home with respect to penalty kicks awarded.