You may have noticed that some analysts have been measuring improvement in the coronavirus statistics by saying that the percentage increase is going down, as measured by new deaths divided by the previous death total. I really shouldn’t have to explain this to anyone who has taken 7th grade arithmetic, but that is mathematical gibberish. Any situation that is stable will appear to decline by using this method, and many situations which are getting worse will appear to get better.
Example One: a stable situation. Let’s imagine a country where exactly one person dies every day for a 30-day month. On day 2, the increase in deaths will be 100%, Day 3 will be 50%, etc. By the time day 1 of the next month arrives, President Salamander can announce, “Look at the great job I’ve done. I brought the rate of increase down to 3%. Just a month ago it was 100%!” I don’t need to show you a graph for this one, The “deaths per day” graph is a straight horizontal line, and the “percentage increase” graph shows a smaller percentage every day, falling quickly at first, then flattening (but still falling slightly) toward the end of the month.
Example Two: a sample situation that gets steadily worse every day. Let’s imagine country B, where the situation just keeps getting worse: one death on day 1, two on day 2, three on day three … until there are 30 on day 30. The daily graph of deaths by day will look like this:
But the percentage of increase in the deaths will look like this:
The percentage increase on day 2 will be 200%, day 3 will be 100%, day 4 will be 67%, etc. The percentage increase will be less every day, although the day-to-day decrease will be very small on the end of the month. (You can see that the graph is nearly flat, but never totally flat as the month ends.) Still, President Pompadour can step up to the podium on the first day of the next month and say, “Look at what a great job I’ve done. I have gotten the increase down to 7% per day. Just a month ago it was 200%.”
So what you want to look at is the graph of deaths per day. That will give you a good picture of how a country is progressing. If you want to get fancy, graph the raw numbers, a 3-day rolling average and a 7-day rolling average to get a better picture of what the trend would look like without anomalies, glitches and flukes. Here’s what those three graphs look like for Italy so far, and I think it gives a good picture of how bad or good things really have been.
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