Abstract: Pigs are species with a strong tendency to live in groups. In wild，they can adjust their group size to accommodate to the environment by themselves. In intensive livestock farms，pigs were stocked in limited pens. And they can not get away from their groups freely. There will be a high risk of abnormal behavior if stoking density is too high. Group size can affect the frequency of aggressiveness，which may further affect the welfare and performance of pigs. In this paper，we discuss the recent researches on group size and floor-space allowance and find that confounding is inevitably introduced when the factors under study include stocking density and group size. In response to this situation we offered some suggestions to avoid or minimize the confounders.

Key words: group size; stocking density; animal welfare; performance; pig