When Is a Measure of Outcomes Like a Coupon for a Diamond Necklace?
For those of you who struggle with the fundamental difference between absolute risk reduction (ARR) versus relative risk reduction (RRR) and their counterparts, absolute and relative risk increase (ARI/RRI), we have always explained that only knowing the RRR or the RRI without other quantitative information about the frequency of events is akin to knowing that a store is having a half-off sale—but when you walk in, you find that they aren’t posting the actual price! And so your question is 50 percent off of what???
You should have the same question greet you whenever you are provided with a relative measure (and if you aren’t told whether the measure is relative or absolute, you may be safer off assuming that it is relative). Below is a link to a great short cartoon that turns the lens a little differently and which might help.
However, we will add that, in our opinion, ARR alone isn’t fully informative either, nor is its kin, the number-needed-to-treat or NNT, and for ARI, the number-needed-to-harm or NNH. A 5 percent reduction in risk may be perceived very differently when “10 people out of a hundred benefit with one intervention compared to 5 with placebo” as compared to a different scenario in which “95 people out of a hundred benefit with one intervention as compared to 90 with placebo.” As a patient, I might be less likely to want to expose myself to side effects if it is highly likely I am going to improve without treatment, for example. Providing this full information–for critically appraised studies that are deemed to be valid–of course, may best provide patients with information that helps them make choices based on their own needs and requirements including their values and preferences.
We think that anyone involved in health care decision-making—including the patient—is best helped by knowing the event rates for each of the groups studied—i.e., the numerators and denominators for the outcome of interest by group which comprise the 4 numbers that make up the 2 by 2 table which is used to calculate many statistics.
Isn’t it great when learning can be fun too! Enjoy!