Incidence plot
This shows that the number of cases increased steadily till the first
week of April and then progressively dropped. This suggests that we had
reached a peak in the number of cases in New Zealand and then there has
been a progressive drop in the number of new cases we continued to find
despite significantly increasing the number of tests. Intuitively, this
would mean that we would find more new cases even with somewhat relaxed
testing criteria that the Ministry of Health used for testing (instead
of tight testing criteria for overseas travel or contact with someone
with history of overseas travel and physical symptoms, now the new
criteria included anyone with suspicious symptoms and signs). We have
fitted an optimised model to find a peak date and patterns of fitted
lines on the existing data. The find_peak() function from the
incidence package returns that the peak was reached around 4th April,
2020.
Next, we fitted an optimised Weibull regression model for the data using
the incidence package and ran the regression model against time
with no other coviariates.
Results of the regression
model
In the model, we regressed log-incidence over time, splitting the dates
into two halves: one before 4th April, 2020, and the ones after 4th
April, 2020. Each of these graphs were exponential graphs, and had
different growth rates. We obtained the following statistics from the
regression model: