In my previous blog entry, I tried to explain why absolute risk reductions was the right number to look at. The trouble is absolute reduction is expressed as a percentage and people generally hate percentages and usually need to do some further mental gymnastics to process the information. So statisticians in the 1980’s came up … Read more
Suppose you see a headline that says something like: “50% of our 100,000 new Covid cases were among the vaccinated”? Should you be concerned that the vaccine isn’t working anymore? The answer is: absolutely not – well, not without a lot more information. This statement is an example of using numbers to confuse rather than … Read more
As I write this more than the equivalent of a 9/11 catastrophe happened yesterday (12/10). It’s horrible and it’s going to get worse. The head of the CDC predicts this level of deaths for the next 60-90 days and even the conservative model used by the IHME says it is likely we will have more … Read more
You have been probably seeing a lot of polls lately. They all end by saying something like “we sampled 1,000 people and our margin of error is 3.8% or 4.5%” or some other weird percentage. I thought I would take some time to explain where this number comes from and what it means. I want … Read more
Herd immunity is all in the news but in many cases the news media is not really describing the concept clearly. While technically the concept of herd immunity derives from mathematical models of disease transmission, when you drill down, it is just generalizing common-sense thinking about disease transmission. Let’s start by imagining only one person … Read more
“”Wisconsin is now experiencing unprecedented, near-exponential growth of the number of COVID-19 cases in our state,” said Governor Tony Evers in a video message.“ Dear Governor Evers, “Near exponential growth” is not a thing. You either have exponential growth or you don’t – and I’m sorry to tell you, you do. I promise I’ll write … Read more
I love back of envelope calculations and I love Rachel Maddow, she is so bright and usually does her homework. Yesterday, not so much. In this segment: http://www.msnbc.com/rachel-maddow/watch/math-on-trump-covid-strategy-has-millions-dying-before-it-works-91958341539 she completely screws up a back of envelope calculation on how many deaths might happen before we get to herd immunity. She starts out correctly: if you … Read more
In a previous blog I said: “you compare the number of people getting the disease or getting severely ill in the vaccinated and placebo group, looking for a statistically significant difference.” and some people asked me is that just an expert’s way of saying “the results are obviously different.” Actually no, it is much more … Read more
Understanding the difference between these two concepts is vital to understanding the results of clinical trials. Heck, it’s vital to understanding just about every intervention in medicine. Get your patient to stop smoking and (obviously) both their absolute risk and relative risk of getting lung cancer drops dramatically. But the numbers are quite different, and … Read more
Some people asked me for the official definition of vaccine efficacy (or VE as it is usually abbreviated). As I suspect you might be thinking, it’s a fraction expressed as a percent. For example, suppose you had 300 cases in the unvaccinated group and 100 cases in the vaccinated group. The VE is then gotten … Read more