Sunday, January 10, 2010

Urban Dynamics Review: Part two

Part two - Conceptual Issues

Aggregation and Definition: The Underemployed, A Case Study by Michael R. Goodman
The underemployed in Forrester's Urban Dynamics model is not the same as defined by census data. We have to be careful about definitions.

Business Structures and Economic Activity in Urban Dynamics by Nathaniel Mass
Some clarifications about business construction. "New-enterprise buildings are constructed predominantly by established firms that are expanding their production capacity and employment levels" not start-up ventures. Actually, young firms are usually in declining-industry structures. "New enterprise structure may be either newly constructed or newly renovated, as long as...employment densities in the building are comparatively high, and the building's tax contribution to the city is large because of its high assessed value."

Turnover in a building happens for several reasons. One is that "changing technology imposes changing requirements for headroom, floor weight capacity, power, and lighting, and work-flow patterns. Turnover rate also depends on...the business climate within the community, taxes, labor, and manager availability, and the socioeconomic character of the urban area...High business turnover rates promote rapid obsolescence of industrial structures because new owners often use the space less efficiently, employee density decreases, and the building becomes more technologically obsolescent."

Dynamic Migration Models by Michael W. Laird
This contrasts the systems dynamics model for migration with the conventional model for migration, which is the gravity model. The main difference is the calculations for attractiveness that are dynamic. The gravity model implies that migration depends only on the sizes of the populations and the distance between places. It mentions the work of I. S. Lowry, who includes an attractiveness factor, but it is not dynamics, although he notes the feedback nature of attractiveness.

Issues of Empirical Support for the Migration Formulations in Urban Dynamics by Michael R. Goodman and Peter M. Senge
This article is interesting because it discusses some difficulties in measuring correlation in real empirical data, making it difficult to verify the causal links in system dynamics models. It discusses four issues.
1. Limitations on inferences which can be drawn from statistical techniques in the face of the restricted time span and range of existing migration data
2. Problems in interpreting parameter estimates derived from cross-sectional data analysis
3. Difficulties in estimating nonlinear relationships containing significant time delays
4. Problems in drawing statistical inferences regarding the error process of a continuous dynamic system.

Metropolitan Population growth, Land Area, and the Urban Dynamics Model by Wilbert Wils
Wils compares the urban dynamics model with actual data of population growth in Massachusetts where "vigorous expansion initially occurs in the central city, but growth later moves outward to the periphery of the city where population densities are lower." This result almost seems like common sense. One distinction from other theories, though, is that in the urban dynamics model, urban decay in the central city precedes expansion rather being caused by expansion.

Urban Dynamics and Its Critics by Louis Edward Alfeld
This is mainly a defense of urban dynamics. Most critics focus on the specific recommendations without addressing fixes to the model with most criticism directed at how the recommendations are bad for the poor and how there is not enough empirical data.

Understanding Urban Dynamics: An Analysis of Garn's Interpretation by Alan K. Graham
This is a response to analysis and criticisms of Harvey Garn of the Urban Institute. Garn was concerned that the urban dynamics recommendations might just be keeping the poor out, make the city better but the nation worse, and that the model neglects the impact of the city-suburb interaction.

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