In this post we look at using 7-day moving averages to estimate the midpoint and total number of COVID-19 cases in New Zealand.
A 7-day moving average adds together the number of new cases for the last seven days then divides the total by seven.
Below is a bar-chart for NZ cases:
We expect the midpoint to occur around 31 March or 1 April.
This was however not easy to visualise when we were at the beginning of April.
Below we have a graph of the 7-day moving averages and the 7-day moving averages for these.
On the blue curve we see a trough around 3-4 April and a peak on the red curve (of the moving averages for the blue curve) around 7-8 April.
We notice that the red curve lags behind the blue curve by four days.
We may have anticipated this because, using a 7-day moving average, each day is at the middle of the moving average on the fourth day.
Hence we also expect the blue curve to also lag behind the actual number of cases by up to four days. We need to look at the bar chart to confirm this.
This as we expected gives a midpoint of 31 March to 1 April (Day 33 and Day 34).
Now we look at the graph for the cumulative number of cases:
The cumulative number of cases for these days (33 and 34) are 647 and 708 giving a total for these two days of 1355 cases.
We add on 10% to allow for a long tail, giving 1490 cases (rounded to 1500 in previous posts), very close to New Zealand current total of 1489 cases.
In future posts we may look at Australia, U.S.A. and the U.K.
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