created with NetLogo
view/download model file: RAndPrefAttachment.nlogo
In some networks, a few "hubs" have lots of connections, while everybody else only has a few. This model shows one way such networks can arise.
Such networks can be found in a surprisingly large range of real world situations, ranging from the connections between websites to the collaborations between actors.
This model generates these networks by a process of "preferential attachment", in which new network members prefer to make a connection to the more popular existing members.
The model starts with 5nodes connected in a cycle.
At each step, a new node is added. A new node picks m existing nodes to connect to randomly, but with some bias. If (gamma = 0) a node's chance of being selected is directly proportional to the number of connections it already has, or its "degree." This is the mechanism which is called "preferential attachment." If gamma = 1, every node that is already in the network has an equal probability of getting linked to be the new node. For intermediate values of gamma, the link will be preferential a fraction (1-gamma) of the time.
Pressing the GO ONCE button adds one new node. To continuously add nodes, press GO.
The LAYOUT? switch controls whether or not the layout procedure is run. This procedure attempts to move the nodes around to make the structure of the network easier to see.
The PLOT? switch turns off the plots which speeds up the model.
The m slider determines how many edges each node comes in with.
The gamma slider determines the proportion of time the linking is random (not preferential).
The RESIZE-NODES button will make all of the nodes take on a size representative of their degree distribution. If you press it again the nodes will return to equal size.
If you want the model to run faster, you can turn off the LAYOUT? and PLOT? switches and/or freeze the view (using the on/off button in the control strip over the view). The LAYOUT? switch has the greatest effect on the speed of the model.
If you have LAYOUT? switched off, and then want the network to have a more appealing layout, press the REDO-LAYOUT button which will run the layout-step procedure until you press the button again. You can press REDO-LAYOUT at any time even if you had LAYOUT? switched on and it will try to make the network easier to see.
This model is based on:
Albert-Laszlo Barabasi. Linked: The New Science of Networks, Perseus Publishing, Cambridge, Massachusetts, pages 79-92.
For a more technical treatment, see:
Albert-Laszlo Barabasi & Reka Albert. Emergence of Scaling in Random Networks, Science, Vol 286, Issue 5439, 15 October 1999, pages 509-512.
Barabasi's webpage has additional information at: http://www.nd.edu/~alb/
The layout algorithm is based on the Fruchterman-Reingold layout algorithm. More information about this algorithm can be obtained at: http://citeseer.ist.psu.edu/fruchterman91graph.html.
For a model similar to the one described in the first extension, please consult:
W. Brian Arthur, "Urban Systems and Historical Path-Dependence", Chapt. 4 in Urban systems and Infrastructure, J. Ausubel and R. Herman (eds.), National Academy of Sciences, Washington, D.C., 1988.
To refer to this model in academic publications, please use: Wilensky, U. (2005). NetLogo Preferential Attachment model. http://ccl.northwestern.edu/netlogo/models/PreferentialAttachment. Center for Connected Learning and Computer-Based Modeling, Northwestern University, Evanston, IL.