period. A large number of overlapping cylinders of different size and shape, together covering the entire study area were
constructed, where each cylinder represented as a potential cluster. The likelihood ratio test statistic was calculated in the
same way as for the purely spatial scan statistic for calculating the likelihood for each window. Initial analysis was conducted
including maximum spatial cluster size of 50 of the total number of cases in the temporal window.
Sensitivity analysis assessment:
Comparative sensitivity analyses were performed for the purely spatial analysis using
different maximum spatial cluster sizes. Comparative sensitivity analyses were also performed for the space-time analysis using a
range of cluster sizes and circle radii. This is because the default setting of 50 used by SaTScan seemed unrealistic as indicated
by other authors Chen et al., 2008. Furthermore, climate and landscape characteristics are believed to strongly influence
dengue transmission, and if a large population threshold 30 was used numerous functional ecological zones controlled
by climate and landscape factors would be crossed by the area within that threshold. Attempts were made to find the most
meaningful population thresholds from the range tested by analysing quantitatively and visualizing the resulting clusters.
3. RESULTS AND DISCUSSION
3.1 Epidemic curves and outbreaks
Figure 1 shows the epidemic pattern of dengue cases and incidence rates Figure: 1 during 1993-2012. Dengue
transmission fluctuated during the study period with two major epidemic peaks in 2003 and 2009 653 cases and 945 cases,
respectively. Peaks in incidence cases coincided with high monthly numbers of SLA areas with dengue cases.
50 100
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Incidence rate
Figure 1: The epidemic pattern of dengue monthly cases and annual incidence rates
Table 1 indicates the descriptive statistics for dengue incidence rates among SLAs in northern Queensland for four time periods
Period 1, period 2, period 3 and period 4. There is a clear trend of geographic expansion of dengue transmission in northern
Queensland through periods 1 and 2 with mean incidence rates equal to 10.2 and 27.1, respectively.
Period Mean
SD Min
Q 1 Med
Q 3 Max
1 34.9
15.3 1.5
23.4 39.3
48.1 61.5
2 10.2
11.6 3.2
5.4 5.46
9.4 61.5
3 27.1
55.4 1.5
5.4 13.6
27.6 549.4
4 12.3
30.7 1.5
4.6 5.4
13.6 671.1
Table 1: Descriptive statistics of dengue incidence rates among statistical local areas in northern Queensland, 1993-2012
3.2 Spatial autocorrelation
Table 2 reveals a significant positive spatial autocorrelation of dengue incidence for all four time periods, where Moran’s
I
value was 0.036 p0.003 during period 1, 0.061 p0.003 during period 2, 0.105 p0.001 during period 3 and 0.250
p0.003 during period 4. This indicates that nearby SLAs tend to have more similar baseline incidence rates than those further
apart.
Study period
Moran’s I
Expected Moran’s I
p-value 1
0.0367 -0.0021
0.003 2
0.0617 -0.0022
0.003 3
0.1054 -0.0021
0.001 4
0.2508 -0.0021
0.003 Table 2:
Spatial autocorrelation analysis
3.3 Spatial clustering
