One of the more difficult problems in modeling transmission of diseases is to model the mixing processes that lead to disease-transmitting contacts between individuals. The most commonly used assumption is that individuals make contacts at random with others in the population. In terms of contacts between subgroups that is commonly called proportional mixing. There are a number of other more complicated ways to model mixing that allow for the possibility of varying within group contact rates as compared with contact rates with other groups. Two of the most versatile are called preferred mixing and structured mixing. In preferred mixing, one can reserve an arbitrary fraction of a group's contacts for within-group mixing; all non-reserved contacts are then subject to proportional mixing. Structured mixing takes into account that most contacts occur in or are initiated in particular gathering places or activities. An arbitrary fraction of a group's contacts is allocated to each activity; the mixing at each activity is then assumed to be a proportional mixing. The details are beyond this introductory treatment, but can be found in Ch. 7 of (2).