can be understood to be ones in which individuals exchange explicit commodities (e.g. labour for wage, service for payment, etc.). Examples include study groups, sports teams, schoolmates, attorney-client, doctor-patient, coworkers, etc. Cooley had made the distinction between primary and secondary groups, by noting that the term for the latter refers to relationships that generally develop later in life, likely with much less influence on one’s identity than primary groups. Collectives A collective is a large group of individuals whose relationships to one another are loosely formed, spontaneous, and brief. Members are generally connected through performing similar actions or possessing similar outlooks. As they only exist for a very brief period of time, it is very easy for an out-group member to become an in-group member and vice versa. Examples of collectives include audiences to a show, bystanders, people at the park, etc. Categories Categories are characterized by an aggregate of individuals who share something in common, but only become groups when their similarities have social implications. Categories can appear to be higher in entitativity and essentialism than primary, secondary, and collective groups. This group is generally the largest type of such, where members can be either permanently or temporarily in-group. Categories can include individuals with the same ethnicity, gender, religion, or nationality. For example, Torontonians, women, and gamers can all be characterized as categories. Campbell (1958) famously defines entitativity as the extent to which collections of individuals are perceived to be a group. The degree of entitativity that a group has is influenced by whether a collection of individuals experience the same fate, display similarities, and are close in proximity. If individuals believe that a group is high in entitativity, then they are likely to believe that the group has unchanging characteristics that are essential to the group, known as essentialism. Reference groups A reference group is a group to which an individual or another group is compared, used by sociologists in reference to any group that is used by an individual as a standard for evaluating themselves and their own behavior. More simply, as explained by Thompson and Hickey (2005), such groups are ones "that people refer to when evaluating their [own] qualities, circumstances, attitudes, values and behaviors." Reference groups are used in order to evaluate and determine the nature of a given individual or other group's characteristics and sociological attributes. It is the group to which the individual