have these within-cell numbers yet, although we have the row and column totals. With data organized this way, our task is to fill in the cells for tables headed t = 1 through say t = n. Actually, from home interview travel survey data and attraction analysis we have the cell information for t = 1. The data are a sample, so we generalize the sample to the universe. The techniques used for zonal interchange analysis explore the empirical rule that fits the t = 1 data. That rule is then used to generate cell data for t = 2, t = 3, t = 4, etc., to t = n. The first technique developed to model zonal interchange involves a model such as this: where: : trips from i to j. : trips from i, as per our generation analysis : trips attracted to j, according to generation analysis : travel cost friction factor, say = : Calibration parameter Zone i generates T i trips; how many will go to zone j? That depends on the attractiveness of j compared to the attractiveness of all places; attractiveness is tempered by the distance a zone is from zone i. We compute the fraction comparing j to all places and multiply T ;i by it. The rule is often of a gravity form: where: : populations of i and j : parameters But in the zonal interchange mode, we use numbers related to trip origins (T ;i) and trip destinations (T ;j) rather than populations. There are many model forms because we may use weights and special calibration parameters, e.g., one could write say: or where: a, b, c, d are parameters : travel cost (e.g. distance, money, time) : inbound trips, destinations : outbound trips, origin Gravity model The gravity model illustrates the macroscopic relationships between places (say homes and workplaces). It has long been posited that the interaction between two locations declines with increasing (distance, time, and cost) between them, but is positively associated with the amount of activity at each location (Isard, 1956). In analogy with physics, Reilly (1929) formulated Reilly's law of retail gravitation, and J. Q. Stewart (1948) formulated definitions of demographic gravitation, force, energy, and potential, now called accessibility (Hansen, 1959). The distance decay factor of 1/distance has been updated to a more comprehensive function of generalized cost, which is not necessarily linear - a