Wobbly Numbers

The projection from two days ago had us above 83,000 for today. As I look at the tracker, we’re only at 76,543. That’s good news, and I always get a smile when a number has some interesting pattern in it. It’s too early to tell whether the curve is flattening or if this is just a blip in the numbers. I’m going to keep my projection at 29% growth rate for now, but that’s a bit higher than the last couple of days have been. I’m also going to extend my projection out to April 3rd as we’re getting close to April now. After the projection, I’ll have some comments about why the numbers may be wobbly and unpredictable for a bit.

Here’s the projection. Remember, this is just what we can expect to see if the current growth rate continues. It’s not a prediction that the growth rate will continue.
3/26 76,000
3/28 126,472
3/30 210,461
4/1 350,229
4/3 582,816

So, why do I expect wobbly numbers? There are a lot of reasons. I’ve mentioned before that different areas of the country are adopting different recommendations regarding testing. Some are testing as much as they can, others are testing only as necessary. If a densely populated area adopts one of these recommendations rather than the other, that could dramatically affect the numbers. Here again it is important to remember that these numbers only represent reported cases, not actual cases. The virus doesn’t care whether we test or not, it’s going to spread the same either way. But testing rates affect our KNOWLEDGE of that spread. If we stopped all testing today, the virus would continue just the same, but our reported numbers would stop growing.

This leads to the bigger issue: The US is actually multiple interacting outbreaks rather than a single outbreak. To see why this matters, let’s look at the first picture, a graphic from the Johns Hopkins tracker that I posted back on the 19th.

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The orange dots are the cases in China, the yellow dots are the cases in the rest of the world. You can just ignore the green dots for this discussion. Notice that there’s a period of time where both of these lines seem fairly flat. It’s true that China had drastically slowed its growth rate at this point. However, what looks like a low growth rate outside of China is actually a trick of the scale. If you look at the second picture, you can see that the growth rate of the yellow curve was fairly stable through this period and maybe even a bit higher than at other periods.

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The trick is that in this period China’s numbers were so much higher than the rest of the world’s that even a small percentage increase in China’s numbers completely dwarfed a larger percentage increase in the rest of the world’s. If we look at a graph for the entire world, we get the third picture.

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Notice that flat area in the middle. China’s numbers were so high, that even though the rest of the world was growing rapidly, it doesn’t really show in the overall growth rate.

Let’s apply that to the US. Right now, NY is the hot spot. NY accounts for approximately half of the reported cases in the US. Because of this, any change in NY’s rate of growth has an outsized impact on US numbers. As NY rates begin to slow, the rates for the US as a whole also slow. But this hides the growth in smaller, but faster growing, areas. For example, Louisiana and Michigan are both growing faster than New York, but their growth is hidden by New York’s huge numbers. As NY hopefully slows, we may see a lull in the overall numbers even when other states are facing crushing growth rates. If we aren’t careful, another state will just take the lead and we’ll start rising quickly again. This is what we saw in the third picture with China compared to the rest of the world.

So, as the individual local and state outbreaks change places in which has the most outbreaks, we may see periods where the national growth rates get better, then worse, then better again. Wobbly numbers.

It’s important to understand this as a swarm of individual outbreaks so that we don’t get a false sense of security as the national growth numbers go up and down. Fortunately, many governors and local officials seem to be aware of this and are taking strong actions to limit growth even before their numbers are high enough to be noticeable at the national level. That’s exactly the right approach for this type of situation.

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