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NetLogo Models Library:
This model explores the concept of path dependence as explained by W. Brian Arthur in his paper, "Urban Systems and Path Dependence" and several other of his papers. Essentially firms walk around a landscape looking for a place to settle. Several mechanisms of how they decide where to locate can be simulated, and the results examined.
Each firm searches for a place to locate. They do this by taking the quality of the current location and the number of firms already located there. They use this to determine the probability of locating at the current location. They then generate a random number, if this number is less than the probability they settle down in the current location.
INITIAL-FIRMS - Controls the number of initial firms in the model
DISTRIBUTE? - Determines whether or not the initial firms are distributed to different locations
SETUP - Starts the model
GO - Runs the model
DISTURB - Tells all of the firms to look for a new location to settle
REGION-SIZE - This specifies how regional qualities influence the current location. At 1, only the current patch's quality is taken into account, as this number is increased the quality of more and more nearby patches is taken into account.
INCREASING-RETURNS - This controls the trade-off between quality and number of firms currently located. If it is set to 1.0 then the only thing that determines whether or not a firm settles in the current location is the number of other firms there. If it is set to 0.0 then the only thing that determines whether or not a firm settles there is the quality of the location.
ATTEMPTS - This specifies the number of patches a firm will visit each turn in determining whether or not to settle down.
BIRTH? - If this is set to true then new firms are created every turn.
If INCREASING-RETURNS is set high then the quality of the winning location may not be very high. If INCREASING-RETURNS is set low then the quality of the winning location will almost always be high. This can be seen in the monitor.
The patches are colored according to the quality of their location. Black patches are low quality, white patches are high quality and green patches are in the middle.
The size of the person shape on a patch indicates the number of firms located at that patch.
Try running the model with both low and high values of INCREASING-RETURNS. Try changing the value and then pressing DISTURB.
Try different sizes of regions and see how that affects the distribution of results.
The current function to determine the probability of settling is an additive function. Try implementing a multiplicative trade-off function.
In Arthur's original paper, his model had different types of firms; this implementation has only one type of firm. Expand the model to have different firm types and different qualities that would be useful for each firm type. In addition Arthur had different probabilities of different firm types being created. This would also make an interesting extension.
Arthur's original model used global knowledge of the state of the world to decide where each firm would settle, here we take advantage of the fact that firms only have local information and have them wander around the landscape.
This model is related to all of the other models in the "Urban Suite".
In particular, it is demonstrating the same concept as the "Urban Suite - Positive Feedback" model, although that model focuses on demonstrating the principle of increasing returns with utmost simplicity, whereas the current model is a little richer in content.
Also, the Segregation library model in NetLogo also explores how individuals decide where to locate but based on very different preferences.
This model is loosely based on the model presented in this paper: Arthur, W.B. (1988), "Urban systems and historical path dependence", in Ausubel, J.H., Herman, R. (Eds), Cities and their Vital Systems: Infrastructure, Past, Present and Future, National Academy Press, Washington, DC, pp.85-97.
Thanks to Bill Rand for his work on this model.
The Urban Suite models were developed as part of the Procedural Modeling of Cities project, under the sponsorship of NSF ITR award 0326542, Electronic Arts & Maxis.
Please see the project web site ( http://ccl.northwestern.edu/cities/ ) for more information.
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Copyright 2007 Uri Wilensky.
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