Collectives

Do the individuals form or belong to aggregations that affect, and are affected by, the individuals?

Collectives are aggregations of agents that affect, and are affected by, the agents. They can be an important intermediate level of organization in an ABM; examples include social groups, fish schools and bird flocks, and human networks and organizations. If the agents in a model can belong to aggregations, and those aggregations have characteristics that are different from those of agents but depend on the agents belonging to them, and the member agents are affected by the characteristics of the aggregations, then the aggregations should be described here as collectives. Collectives can be modeled in two ways. First, they can be represented entirely as an emergent property of the agents, such as a flock of birds that assembles as a result of the flight rules given to the simulated birds. In this case, the collectives are not explicitly represented in the model: they do not have state variables or behaviors of their own. Second (and more common) is representing collectives explicitly as a type of entity in the model that does have state variables and its own behaviors. Social groups of animals or people (dog packs, political parties) have been represented this way. Describe: • Any collectives that are in the model. • Whether the collectives are modeled as emerging entirely from agent behaviors, or instead as explicit entities. • In overview, how the collectives interact with each other and the agents to drive the behaviors of the entire system. (The details of these interactions will appear in other ODD elements.)

In ITC Evaluation Model:

• At initializations there are no collectives, only individual agents
• During simulation, an agent can help another agent to evacuate. In such a case these agents will have the same evacuation behavior and may be regarded as a group. 
• This model includes two kinds of collectives, groups of wild dogs that strongly affect the individual dogs and are strongly affected by the individuals. The collectives are represented as specific kinds of model entity with their own state variables and behaviors. These entities are called packs and disperser groups; the entities and their state variables are defined above at Entities, State Variables, and Scales. These collectives are included in the model because wild dogs have many cooperative behaviors and make decisions critical to the population’s abundance and persistence in ways that depend on their group’s state. It is much easier to model cooperative behavior and collective decisions as behaviors of collective entities than as behaviors of the individual dogs.
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