# Absolute Risk and Relative Risk

The absolute risk of developing a disease can be expressed as chance (such as 1 in 10), as a percentage (such as 10 %), or as a decimal (such as 0.1). They are not made in comparison to anything else and are simply probabilities of an event occurring. However, in research comparison of two groups is often necessary in order to understand if a particular behaviour or food has a modulating effect on the risk of developing a disease, and in this case the relative risk is used. Relative risk is therefore an expression of how risk changes with a particular behaviour, and is expressed as the absolute risk of developing the disease in the exposed (treatment) group, over the absolute risk of developing the disease in the non-exposed (control) group. Importantly, relative risk tells the reader nothing about the actual risk, it is just a measure of how risk changes.

For example, in a trial magnesium supplements are given to the treatment groups, and a placebo given to a control group. Both groups are followed over time and at the end of the trial, the total number of myocardial infarctions in each group is counted. In the treatment group 2 of the 20 subjects experienced a myocardial infarction, and so the absolute risk of a myocardial infarction in this group is 2/20 = 0.1 (a 10% absolute risk of an event). In the control group 4 of the 20 subjects experienced a myocardial infarction, and so the absolute risk of a myocardial infarction in this group is 4/20 = 0.2 (a 20% absolute risk of an event). To calculate the relative risk we take the absolute risk in the treatment group (0.1) and divide it by the absolute risk in the control group (0.2); 0.1/0.2 = 0.5 (50%). Magnesium therefore halves the risk of an event (a relative risk of 0.5).

Note that if in our example 5 subjects in the treatment group had experienced a myocardial infarction, and in the control group 10 had experienced a myocardial infarction, the relative risk would still be 0.5. This is because relative risk tells us nothing about absolute risk. Changes in absolute risk can be expressed, but they tend to be smaller. In our example, the change in absolute risk between the treatment group (0.1; 10 %) and the control group (0.2; 20 %) is therefore 10%. Absolute risk can therefore be used in papers to exaggerate the benefits of treatment and care should be taken by the reader to be aware of this. For example, relative risk ratios have been used extensively in many drug trials including the statins, and this has given the impression of a greater effect than is really present. In fact, when the absolute risk is considered, and total mortality is the outcome, the statin drugs show no benefits whatsoever.

In research, relative risk is often abbreviated to RR. A relative risk of 1 means there is no difference in risk between the two groups for developing the disease. A relative risk of < 1 means the event is less likely to occur in the exposed group than in the non-exposed group (e.g. a relative risk of 0.5 means risk is halved). A relative risk of > 1 means the event is more likely to occur in the exposed group than in the non-exposed group (e.g. a relative risk of 4 means the risk is increased 4 times). Absolute risk tends be found in introductions and review papers that are highlighting the absolute risk of a population developing a disease (usually from national statistics and large scale epidemiological studies). Relative risk is also found frequently in epidemiological research papers interested in associations between diet and disease, as well as clinical trials investigating the effects of a food or supplement on a metabolic parameter.