According to Wikipedia:
Normative influence is a function of social impact theory (Lantane, 1981), which has three components. A group’s strength is how important the group is to you. Groups we value generally have more social influence. Immediacy is how close the group is to you in time and space when the influence is taking place. The number of people in the group has a surprising effect. As the number increases, each person has less of an impact. Psychologists have constructed a mathematical model using these three factors and are able to predict the amount of conformity that occurs with some degree of accuracy (Latane & Bourgeois, 2001).