COVID-19 Daily Update for Yesterday, Sunday, 8/23/2020:

World Wide Trends in Brief: 

In today's installment of "is it signal or is it noise", let's look at the World Wide daily change in the number of active cases. 

Active Cases are defined as the number of cases - number recovered. So the daily change in active cases, is the number of new cases that day, minus the number of people who have recovered that day. In other words, it's the orange line, minus the green line below:

When it's positive (when the orange line is above the green line) the number of active cases world wide are growing. When it's negative (when the green line is above the orange line) the number of active cases world wide is shrinking. 

When we plot that difference, we see that with a 7 day trend, the daily change in the number of active cases shrank for a while, but is now starting to grow again: 

However, with a 14 day average, the daily change in the number of active cases is still smoothly falling:

Remember that even if this line is heading downward, as long as it is positive, things are getting worse. It needs to be negative before things are getting better. That being said, are things getting worse at a decreasing rate (14 day average)? Or... are things starting to get worse at an accelerating rate (7 day average)? 

This question (is it signal, or is it noise) is one of the KEY questions of statistics. And it is not at all easy to answer. 

Given that there are strong "weekend" effects, we know we should be taking moving averages of the data in increments of 7. (With less than that you start to see weekly fluctuations that make the interpretation even more difficult). However, a 7 day average is potentially too small of a chunk, and it can be impacted by noise which makes you think you are seeing a new "trend" when you aren't. However, the 14 day average is too large, because it can hide trends that are, in fact, real, and you have to wait at least two weeks to be sure that something real has changed. Neither is ideal... and I would prefer something like a 10 day average. But the weekend effects make that even more problematic than either the 7 or the 14 day averages. 

This is a BAD idea, but I will show you what it looks like anyway: 

It is POSSIBLE that the right thing to do would be to create a complicated model of the weekend effect, then take it out of the data, correcting for it... then take a 10 day average, and see what the trend says. But the strength of the weekend effect depends on which country is reporting the most cases at the time (some have a stronger weekend effect than others) so correcting for this is a moving target! It's potentially impossible to really get right. 

So instead... we are just going to have to wait... watch... and see where this trend goes in the next week or so. 

Did I mention that I hate waiting?

via GIPHY