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 as you will see absolute risk reduction is always a smaller number then relative risk reduction, sometimes a dramatically smaller number. Confusing them is a horrible mistake for a medical professional.
I’d venture to say that every doctor, epidemiologist, MPh is supposed to understand the difference between the two concepts just like they should understand the difference between other basic concepts like systolic vs diastolic blood pressure. So, I find it crazy that FDA Commissioner Stephen Hahn M.D. forgot or confused them. In any case, he was forced to issue an incredible mea culpa after the crazy press conference on convalescent plasma a few days ago.
“I have been criticized for remarks I made Sunday night about the benefits of convalescent plasma. The criticism is entirely justified,” the commissioner said in a string of tweets. “What I should have said better is that the data show a relative risk reduction not an absolute risk reduction.”
Anyway, understanding the difference between these two concepts is not only important to understanding fatuous press releases from drug companies or you deciding if you want to undergo a specific treatment, it’s also important in dealing with risk assessment in all areas, so I thought I would write a blog explaining the difference.
Absolute risk reduction is the easiest to understand. Usually, it is easiest to think of this as the percentage you lessen the bad crap from happening after you do something good like regularly taking your medicine or stopping smoking – or getting a vaccine. As an example, suppose a new drug drops the percentage of patients dying from 15% of the patients to 12%. Then the absolute risk reduction is 3%.
The trouble with absolute risk reduction is that, especially for drug interventions, it is usually a small number and so hard to completely understand what it means. And what’s worse is that the behavioral economists have found that we seem wired to focus on the small (3% reduction) as the measure of the success of the intervention, missing the forest for the trees. It seems a lot of people would just say “3% doesn’t seem like such a big deal.”
And that is really really bad because any drug with this kind of absolute reduction in deaths is actually a pretty good drug. Enter relative risk reduction which does allow you to focus on the forest and not the trees. Yes, relative risk reduction is always going to result in more spectacular numbers and so, using it requires caution. But heck, if it works to get a patient to take their medicine it can be a good thing- sometimes.
Going back to our hypothetical drug, the relative risk reduction in deaths is a percentage but it is actually a little tricky to conceptualize. So seeing the calculation first is easier than understanding the definition! All you have to do is put the absolute risk reduction (3%) in the top of the fraction and the baseline risk (15%) in the bottom
Relative Risk Reduction = (3%/15%) or 20%!
Now the definition may make some sense: relative risk reduction is: “the percentage of baseline risk that is removed as a result of a treatment”.
The problem with only using relative risk reduction in an explanation is that it depends completely on the baseline. Without knowing the baseline and its size, relative risk reduction numbers alone are often useless.
Here are some examples. Suppose the risk of getting sick is 90% and a vaccine brings this down to 50%. The absolute risk reduction is 40%. So the relative risk reduction is:
40%/90% or about 45%
That’s an OK (not great) reduction but it is going to be statistically significant for a vaccine trial which tend to be large!
But now suppose the risk of dying from a disease is 2% and a drug drops this to 1% in a trial. Then the relative risk reduction is even better:
1%/2% or 50%
but this may not be statistically significant depending on the size of a trial even though this is a larger relative risk reduction than our vaccine.
So let me end by reiterating that public health officials need to be especially careful when citing risk statistics to clearly say which they are telling you about. While confusing absolute reduction and relative reduction rates is a mortal sin, citing a relative rate reduction without a baseline is a lesser but still bad, sin. Fail to report complete information or showcase headline numbers without context because you want to have the numbers look “great”, and you will end up, like FDA commission Hahn did, looking like an idiot at a press conference.