Allow quantity relationships as well as existing proportional relationships

[gregorbj]

Currently all causal relationships in FSDM are proportions. Change is measured proportionally and the proportional change in the posterior concept is proportional to the proportional change in the anterior concept. This works well for many types of causal relationships. For example, the proportional change in vehicle miles traveled is proportional to the proportional change in development density. However, this approach does not work when the quantity of change in the posterior concept is related to the quantity of change in the anterior concept. For example, in a model of traffic shifting from one portion of the road system to another portions the change in the traffic is going to be related to the quantity of change, not to proportional changes. Say if 10,000 trips shift from arterials to freeways, the change is the 10,000 trips, not some proportional change in arterial traffic. Adding this capability will also enable users to have correct accounting of values.

This can be implemented by establishing relationship types in the definition of the model. The calculation would identify the type and then apply the appropriate procedure for calculating the change in the posterior concept as a function of the change in the anterior concept. Establishing new types would also enable other types of relationships to the mix with minimal reprogramming.