Geography Reference
In-Depth Information
Similarly, the district's space-distance can be defined as the distance between any
centroids of two districts, denoted as
U
and
V
,asinEq.
15.4
District's Space-distance
U;V
D j
location
u
location
v
j
; where
u
2
U and
v
2
V
(15.4)
15.2.2
Stage 1: Diagram of Space-Time Distances
The diagram of space-time distances can be established by counting the cumulative
pairs of points calculated at a given space-distance and time-distance. The diagram
is a two-dimensional contour plot; the x-axis is the time-distance, and the y-axis
is the space-distance. The values of the contour are the cumulative values of the
space-time pairs
DR(i, j)
, which are defined as in Eq.
15.5
ˇ
ˇ
ˇ
ˇ
.m; n/
.p; q/
ˇ
ˇ
ˇ
ˇ
DR . i ; j /
D
[
RSD.m;n/
D
i
\
[
RTD.p;q/
D
j
;
(15.5)
where
(m,n)
is a pair, with
m
and
n
ranging from 1 to the total number of cases.
RSD
(m,n)
is the rounded spatial distance at row
m
and column
n. RTD (m,n)
is the
rounded temporal distance at row m and column n. Therefore, DR (i,j) is the number
of space-time pairs when
RSD
D
iand
RTD
D
j.
Higher values of the
DR (i,j)
indicate that more cumulative pairs of points were
found at a given space- and time-distance, showing space-time clustering of cases
(Kulldorff and Hjalmars
1999
). The outlier-detection technique is used to determine
the number of cumulative pairs of points that is significant. The threshold in this
study is defined as the third quartile plus 3 times the inter-quartile range (IQR)
(Kvanli et al.
2005
). When the number of cumulative pairs of points was greater
than the “upper fence” value, the pairs of space-time distances were selected and
the spatial distribution of these pairs was mapped.
15.2.3
Stage 2: Analyzing Link-Node Structure of Space-Time
Pairs
We regarded infected cases as nodes and selected space-time pairs of these cases
as links. A link-node network of space-time pairs was established. The network
topology of space-time distances could thus be analyzed further.
In this study, we used two network indicators to measure the risk of epidemic
diffusion. If one node that exists in many pairs represents a person who has
many connections with others in time and space, then he/she could be regarded
as a possible infection source. This concept is identical to degree-based centrality
Search WWH ::
Custom Search