Geography Reference
In-Depth Information
15.2
Methodology
The proposed methodological framework is composed of two procedures for
tracking epidemic diffusion. In the first stage, GIS is used to analyze the week
of onset and the residential location of each confirmed case to establish case-to-
case distances across space and time. Cumulative pairs of points calculated at a
given space-distance and time-distance are counted. The diagram of space-time
distances allows the identification of the space-time clustering of cases. Then, the
space-time pairs of these clustering cases are selected and mapped. In the second
stage, the link-node network based on these pairs is further analyzed. In this paper,
we use the link-node topology of space-time distances to determine the network's
topological indicators, such as the degree centrality scores of each case, which can
be used to identify possible sources of infection (individuals with higher values of
the indicators). Each procedure is described below in detail.
15.2.1
Definitions of Space-Time Distances
Basing our model on the Knox statistic (Knox and Bartlett
1964
; Kulldorff and
Hjalmars
1999
), we used the illness onset time and residential locations of each
infected case to determine case-to-case distances in space and time. The individual's
temporal distance is defined as the difference in the onset week between any two
infected cases, designated as
u
and
v
,asinEq.
15.1
:
Time
distance
u
;
v
D j
InfectTime
u
InfectTime
v
j
;
(15.1)
where InfectTime
u
is the onset time of infected individual
u
. Moreover, when we
use aggregated areal data (e.g., district or county) as the spatial unit for analyzing
diffusion, we can define the district's temporal distance as the difference in the time
of first onset between any two districts, denoted as
U
and
V
,asinEq.
15.2
:
ˇ
ˇ
ˇ
ˇ
.InfectTime
v
/
ˇ
ˇ
ˇ
ˇ
District's Time
distance
U;V
D
.InfectTime
u
/
min
v
2
V
min
u
2
U
:
(15.2)
where
u
2
U
such that InfectTime
u
0
D
min
u
.InfectTime
u
/ and
v
2
V
such that
2
U
InfectTime
v
0
v
2
V
.InfectTime
v
/.
Space-distance is defined as the distance between the locations of any two
infected individuals, denoted as
u
and
v
,asinEq.
15.3
:
D
min
Space-distance
u
;
v
D j
location
u
location
v
j
:
(15.3)
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