Some researchers report relative risk (RR) and odds ratio (OR) when comparing two groups’ outcome after they were exposed to a risk factor. While these concepts can help us make better sense of risk assessment, some people can confuse both terms.
First, risk is the probability of someone getting sick or hurt, while odds is the ratio between those who got sick or hurt and those who didn’t. For example, a study that finds 6 out of 100 runners got a knee injury over a six-month period would say that this population of runners has a 6% chance of getting a knee injury during this period of time.
The odds of getting a knee injury would be 6/94, which is 0.064. Compared to the risk, the odds is very insignificant.
RR takes the existing data and compares the risk of those who were exposed to the risk factor and those who didn’t. In one study, Yamato et al. cited the RR measured is 2.1, which means those who had difficulty carrying a heavy schoolbag have twice the risk of getting back pain than those who don’t—with the baseline of 1.0.
OR is similar to RR but it compares ratios instead of probability. Another study that compared weight perception and back pain found an OR of 2.2, where those who perceived a heavy bag were twice as likely to get back pain than those who didn’t.
However, RR and OR are meaningless if we don’t know how many people were involved in the study. This is where we need the absolute risk to determine the baseline risk. For example, if we say heavy backpacks may increase the likelihood of getting back pain by 50%, that number seems huge. However, if we use absolute risk and reveal that heavy backpacks increase risk of back pain from 1 per 100 students to 2 per 100 students, then we have a different perspective. (That perspective changes further if we increase (1,000) or decrease (10) the sample size, which changes the RR.)
We should also consider that having an increased risk does not guarantee that an illness or injury will happen, and neither does having a decreased risk mean that it will not happen. Given the current pool of data, we can say that carrying a heavy backpack may contribute to back pain, but the risk is quite small compared to other factors, such as mental health, family and social support, parental communication about pain, and personal beliefs and perceptions about pain.
RelativelyRisky on Twitter offer some examples of the differences by using mainstream news reports.
1. Noordzij M, van Diepen M, Caskey FC, Jager KJ. Relative risk versus absolute risk: one cannot be interpreted without the other. Nephrol Dial Transplant. 2017 Apr 1;32(suppl_2):ii13-ii18. doi: 10.1093/ndt/gfw465.