Cross correlation analysis
Table 1 shows results of Spearman Rank contemporaneous correlations
between GTD and incident case numbers for different European countries.
The increasing and decreasing phases of each country’s epidemics were
also examined. For the increasing phase, the correlation was significant
in all countries studied. With cross correlation analyses, in the
majority of cases the correlation was strongest at a median of 0 day
(IQR: 0-1) lag (Figure 1). In other words, GTD was contemporaneous with
incident cases (with the notable exception of Ireland, where it preceded
it by 18 days).
This pattern was also observed when data describing the decreasing phase
of the epidemic was examined. All but two countries had significant
moderate to strong positive correlations between GTD and incident case
numbers (the exception being Sweden and France). With cross correlation
analyses the median lag was also 0 days (IQR: -2.5-0, further details in
the Supplementary material S1).