Tail biting is a multifactorial problem with important welfare as well as economic consequences. Different stakeholders in the pig production chain, such as farmers, consumers and policy makers are interested in the level of tail biting, because tail biting may affect productivity, profit and animal welfare. To help assess the extent to which a housing system is at risk with respect to tail biting we constructed a computer-based decision support system called PIGTAIL. For ‘validation’ the model was run on 50 treatments, i.e. ‘housing systems’ described in 12 scientific papers. Of 77 pairwise comparisons of these ‘housing systems’ 63 comparisons were in accordance with PIGTAIL’s predictions (sign test, P