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Note: If you download the NetLogo application, every model in the Models Library is included. 
This is a 3D version of the NetLogo model Preferential Attachment. 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 two nodes connected by an edge.
At each step, a new node is added. A new node picks an existing node to connect to randomly, but with some bias. More specifically, 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."
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 RESIZENODES 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 REDOLAYOUT button which will run the layoutstep procedure until you press the button again. You can press REDOLAYOUT at any time even if you had LAYOUT? switched on and it will try to make the network easier to see.
The networks that result from running this model are often called "scalefree" or "power law" networks. These are networks in which the distribution of the number of connections of each node is not a normal distribution  instead it follows what is a called a power law distribution. Power law distributions are different from normal distributions in that they do not have a peak at the average, and they are more likely to contain extreme values (see Albert & Barabási 2002 for a further description of the frequency and significance of scalefree networks). Barabási and Albert originally described this mechanism for creating networks, but there are other mechanisms of creating scalefree networks and so the networks created by the mechanism implemented in this model are referred to as BarabásiAlbert scalefree networks.
You can see the degree distribution of the network in this model by looking at the plots. The top plot is a histogram of the degree of each node. The bottom plot shows the same data, but both axes are on a logarithmic scale. When degree distribution follows a power law, it appears as a straight line on the loglog plot. One simple way to think about power laws is that if there is one node with a degree distribution of 1000, then there will be ten nodes with a degree distribution of 100, and 100 nodes with a degree distribution of 10.
Let the model run a little while. How many nodes are "hubs", that is, have many connections? How many have only a few? Does some low degree node ever become a hub? How often?
Turn off the LAYOUT? switch and freeze the view to speed up the model, then allow a large network to form. What is the shape of the histogram in the top plot? What do you see in loglog plot? Notice that the loglog plot is only a straight line for a limited range of values. Why is this? Does the degree to which the loglog plot resembles a straight line grow as you add more node to the network?
Assign an additional attribute to each node. Make the probability of attachment depend on this new attribute as well as on degree. (A bias slider could control how much the attribute influences the decision.)
Can the layout algorithm be improved? Perhaps nodes from different hubs could repel each other more strongly than nodes from the same hub, in order to encourage the hubs to be physically separate in the layout.
There are many ways to graphically display networks. This model uses a common "spring" method where the movement of a node at each time step is the net result of "spring" forces that pulls connected nodes together and repulsion forces that push all the nodes away from each other. This code is in the layoutstep procedure. You can force this code to execute any time by pressing the REDO LAYOUT button, and pressing it again when you are happy with the layout.
See other models in the Networks section of the Models Library, such as Giant Component. The Preferential Attachment model is also implemented in 2D.
See also Network Example, in the Code Examples section.
See also the Path Dependence model in the 2D Urban Suite.
This model is based on: AlbertLászló Barabási (2002). Linked: The New Science of Networks, Perseus Publishing, Cambridge, Massachusetts, pages 7992.
For a more technical treatment, see: AlbertLászló Barabási & Reka Albert (1999). Emergence of Scaling in Random Networks, Science, Vol 286, Issue 5439, 15 October 1999, pages 509512.
Barabási's webpage has additional information at: http://www.barabasilab.com/
For a model similar to the one described in the first extension, please consult: W. Brian Arthur (1988), "Urban Systems and Historical PathDependence", Chapt. 4 in Urban systems and Infrastructure, J. Ausubel and R. Herman (eds.), National Academy of Sciences, Washington, D.C., 1988.
If you mention this model or the NetLogo software in a publication, we ask that you include the citations below.
For the model itself:
Please cite the NetLogo software as:
Copyright 2011 Uri Wilensky.
This work is licensed under the Creative Commons AttributionNonCommercialShareAlike 3.0 License. To view a copy of this license, visit https://creativecommons.org/licenses/byncsa/3.0/ or send a letter to Creative Commons, 559 Nathan Abbott Way, Stanford, California 94305, USA.
Commercial licenses are also available. To inquire about commercial licenses, please contact Uri Wilensky at uri@northwestern.edu.
This is a 3D version of the 2D model Preferential Attachment.
(back to the NetLogo Models Library)