Trends or Noise? A brief review of how to look at data …

ZOMG! TODAY IS ANOTHER RECORD DAILY INCREASE IN COVID CASES AND THE SECOND RECORD IN A ROW!  Or, it’s a leveling off of the increase.  Or … well, what is it?  It sort of depends on how you look at the data.  I’ve spilled lots of electrons on how the “case” data is really problematic, but that’s what people are using for decision making, and it’s closer to what the virus is doing in “real time” (only about two weeks behind reality, vs. the mortality rate, which lags about a month).  So let’s dig in … sorry, no equations.  

First, the raw numbers and “death counters” you are seeing everywhere can be very misleading.  The spread of a virus through a population depends on a lot of factors, but one of the biggest is population density.   You HAVE to normalize these numbers for population for this to make sense.  100 people in New York City is a blip.  100 people in rural Georgia is a catastrophe. Likewise, two months ago when there were over 20,000 new cases a day, they were mostly in New York and New Jersey; today, they are scattered across a dozen states.  So here is what we are seeing in representative states for the rate of new positive tests (people are calling these a “case” but that’s a misnomer).  This graph shows new positives per 10,000 of population.  Yes, there are some negative numbers and zeros due to reporting issues … (SC and LA are really crummy data sets).  Click to embiggen.

So in relative terms, most states are seeing a spread of the virus on the order of 1/3 the rate of what was seen in NY/NJ at its peak.  Note that’s not a good thing … but, again, 30k people infected in NY/NJ (and most of that in the Metro NYC area, population of about 20 million) vs 30k people across the entire US (population 321 million) is a very different thing. But … we are seeing some states (TX, GA, SC) doing much worse, not only moving in the wrong direction, but into the realm where you have to start worrying about the health care system getting stressed.

Let’s look now at the country as a whole:

Notice how flat this curve looks compared to the state level.  That said, there is a disturbing trend over the last 10 days, however, it’s not really the dramatic upward spike that seems to be the perception in some circles. And as I have often said, single day trends, even week trends, can be misleading.  Is today a plateau, followed by a downward trend, or will the rate increase?  My guess is it will wobble a bit higher then trend back down, and the “stable” point nationally over the summer will be around 1 per 10,000 per day in a “somewhat open, but with most people being sensible” paradigm. That’s not as good as the previous somewhat “stable” point of around ~0.75 per 10k for the last two months, but not horrific.  The trick in looking at this data – and creating policy to address the pandemic – is figuring out the trends, with the knowledge that with new positives you are two weeks behind the curve.  The data point for tomorrow depends on what people did 7-14 days ago.

In short: absolute numbers may be dramatic, but it’s the trends in rate per capita that matters.  Speaking of which, absolute hospitalization and death numbers, while vital to gauge the stress on the health care system (which is getting worse in many states, especially where the positive rate is moving above 1.5 per 10,000), aren’t telling you how deadly or dangerous the virus is.  As noted yesterday, the rate of people who are positive that end up in the hospital, and the rate of people who are dying from the virus, is actually decreasing slightly.  This is likely because more people in the 20-50 year old range are the bulk of the new positives.  So while the news coverage has shifted this week, what we are seeing isn’t a dramatic change in the virus.

The trend over the last 10 days is of course being blamed on reopening, and that is a big factor, but more importantly these trends are a direct result of people relaxing basic precautions – especially not wearing masks in public places and gathering inappropriately.  The reopening didn’t have to cause this increase.  In my view it is an urgent call to take more precautions on an individual level. (Update: and, an hour or so after I posted this, guess who said the same thing, but obviously a bit better …)  I don’t think it’s a call to close things down again; our economy is very fragile (likely in an ongoing collapse with the worst yet to come) and just can’t handle that.

Finally, I’m not trying to minimize how bad this is, or get into the partisan poo flinging over it.  As for those saying that the positives are related to testing; the argument that more testing is the reason we are seeing more positives  is “not even wrong.”  But that said, we need to have some context about this pandemic, and chart a careful path between over-reaction and under-reaction.  Sadly, all we seem to be doing is a random weave between the extremes … and that has severely damaged our economy, and maybe even our society as people are increasingly frustrated and angry as pre-existing fault lines are exposed and aggravated.

2 thoughts on “Trends or Noise? A brief review of how to look at data …

  1. I think your essay is a lot ‘opinion’ rather that hard cold fact. The fact that the overall deathrate seems to be flat…doesn’t excuse the stupidity of people who don’t give a shit and endanger others who cannot avoid contracting the virus or in many cases have compromised immune systems. The fact also that Florida, Texas and Arizona, to name 3 culprits, took a decidingly careless attitude toward being typhoid marys or at very least exposing elderly, compromised or infant-children-none of whom can hold out against this virus….is what upsets and angers the rest of us who see spikes in some states where we have extended family. So, in my estimation, over reaction and intense reluctance to give those ‘virus carriers’ or exposed through careless, is absolutely understandable. Personally, if I happen to hear of or see a car from one of those states driving around my small and relatively below ‘flat’ statistics, you can bet I am going to over react!

    • The data is what it is: the infection rate is increasing, the death rate is flat (although, as noted in today’s post, that’s complicated). Those are cold hard facts (within the noise that is in these data sets – which is itself a fact). I agree 100% that several states – and a lot of individuals – are doing some really irresponsible things. But it is also true that over-reaction carries a significant cost (in lives as well as money, by the way). We can’t just swing between the extremes, and shout at each . We have to figure out why people who are under-reacting are under-reacting and get them to act appropriately. Likewise, we have to get people who are over-reacting to try to climb down off the ledge. That is super hard now that those two camps are identified with, and being exploited by, politicians looking to the next election. As for opinion, the under-reaction crowd is being more irresponsible than the over-reaction crowd, but he over-reaction crowd isn’t helping convince them.

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