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residential locations. In calculating inner-migration rates, it is necessary to correct
for the fact that inner-migration rates are defined by arbitrary spatial units (Turner
et al. 1989 ), in that small-area data can be combined at different resolutions. We
measured the inner-migration rates across a series of 29 regular grids ranging from
a coarse one 2 2 of grid cells to a fine-scaled 3 3grid for the entire TCMA. At
finer scales, up to a third of the grid cells fall outside of the seven-county region
given its irregular boundary and are not included in the count because they would
inflate the number of seemingly correct moves.
The simulated inner-migration rates mirror actual rates given by the parcel data,
which indicates that the model captures key relationships between vacant housing
opportunities, move distance distribution, and land-use patterns (Fig. 6.5 a). Both
decision-making strategies—distance-only and distance-plus-direction—produce
inner-migration rates that are close to the actual values. Distance-and-direction
outperforms just distance, as illustrated by the total root mean squared errors, which
compares how well the simulation does against actual moves measured by inner-
migration rates (Fig. 6.5 b).
Syrjala tests offer an advantage over inner-migration rates in that they demon-
strate how well a simulated distribution resembles an actual one (Syrjala 1996 ).
The Syrjala test compares the values of two sets of samples or, in the case of
intraurban migration, destinations tessellated onto a regular grid. The test produces
two measures, a Syrjala statistic and a p value. The Syrjala statistic measures the
differences between the cumulative distribution functions of the two samples. The
smaller the statistic, the closer the two sample distributions, while the p value
indicates the probability that the two samples are from the same population and
spatial distribution. There is no simple analytical solution for p ;instead,itis
calculated through sample permutation and denotes the percentage of randomized
permutations that have bigger Syrjala statistic than the sample. If p is 0.03, for
example, only 3 % of these random distributions are more similar to the distribution
of one sample A than the other sample B, implying the spatial distribution of A is
statistically different from that of B.
a
b
25
0.8
20
15
0.6
Actual
Distance Only
Distance plus Direction
10
0.4
5
Distance Only
Distance plus Direction
0.2
0
0
100
200
300
400
500
600
0
100
200
300
400
500
600
Number of Spatial Units
Number of Spatial Units
Fig. 6.5 Actual and simulated inner-migration rates ( a ) and total RMSE of simulated inner-
migration rates against actual rates ( b )
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