AMP/UserGuide/FromScratch

Creating a new model from scratch: A naive, ground-up tutorial

This tutorial assumes no knowledge of AMP. It is a work in progress, and shows how one naive AMP user navigates through the model creation process. While it is under construction there may be parts that trail off; the AMP developer community will chime in for guidance, and these loose ends will be sewn up, leading to the next challenge.

The Model

The model to be created is a simple one. It is based on a forthcoming paper whose primary author is John W. Pepper (University of Arizona), who built on his previous work by taking a suggestion from Robert Axelrod.

The model is intended to demonstrate that positive and negative assortment can occur among populations comprised of two kinds of individuals both of which, regardless of their own type, show a preference for being near members of one type. The situation is analogous to a condition in which one kind of individual causes some form of environmental degradation that impacts itself and others around it.

The model specification is as follows:

N agents exist in space. Agents are of two kinds, traditionally called Cooperators and Defectors. Agents are able to perceive the agents immediately around them within a radius of r units; r is the same for all agents. Agents are able to discern whether these neighbors are C's or D's. If the number of D's around a given agent is greater than the number of C's, the agent will move to a new, randomly selected location (with uniform probability throughout the space).

Parameters in the model:

• N - the number of agents, usually 100
• DF - the 'Defector Fraction', or the proportion of the N agents that are defectors
• r - the radius agents can 'see'

Additionally there are some other variations that will eventually be performed; I'm not sure how AMP will consider these, and whether they are parts of the same 'model' or would be different 'models'. For now the main issue is space; the original formulation, which we can take as a starting point here, is that space is a continuous 1-D ring with periodic boundaries. Agents are located at points along this ring, which is considered to have length 1 (so agent locations are all values between zero and one).

An additional point: all agents assess their neighborhoods simultaneously; then all agents that choose to move on the basis of this assessment move simultaneously.