Consider the risk or severity of the health problem when estimating the size of expected effects.
Deciding whether the benefits of a health action outweigh the harms depends on the size of the good and bad effects. The size of the effects may depend on how sick you are or how likely you are to get sick without taking the health action.
Explanation
The balance between the benefits and harms of a health action often depends on the likelihood of an individual experiencing a health problem without taking the health action (the baseline risk), or on the severity of the health problem. The balance between the benefits and harms of a health action is more likely to favour the use of the health action by people with a higher baseline risk, or more severe symptoms.
Generally, the benefits of a health action are less for someone with a low baseline risk compared to someone with a high risk. On the other hand, the risk of harms is often the same. Therefore, the benefits and harms of a treatment tend to be more closely balanced for people with a low baseline risk than for someone with a high baseline risk. The same is true for people with more severe symptoms (e.g., pain or depression) compared to people with less severe symptoms.
Uncertainty about someone’s baseline risk can reduce how sure you can be about the size of the benefits and harms and consequently, about the balance between the benefits and harms of a health action.
Example
Researchers have found that taking low-dose aspirin reduces the likelihood of deaths, heart attacks and strokes but slightly increases the likelihood of serious bleeding in the gut. A person who has had a heart attack or stroke, or who has a high likelihood of dying or having another heart attack or stroke in the next five years has a high baseline risk. Because of this high baseline risk, for this person, the beneficial effects of taking low-dose aspirin (reduced deaths, heart attacks, and strokes) are substantially more than the harms (increased serious bleeding in the gut, and minimal inconvenience and cost).
On the other hand, someone 60 years old with a low likelihood of dying or having a heart attack or stroke in the next five years has a low baseline risk. In this person, taking low-dose aspirin will have little beneficial effect on deaths and strokes (a slight reduction of an already low level) and a slight increase in the likelihood of serious bleeding in the gut. Consequently, the benefits and harms of low-dose aspirin are closely balanced for someone in this situation. Some people in this situation would choose to take aspirin, and some would not.
Remember: When making decisions about health actions, consider the likelihood of getting the health problem and the severity of symptoms without taking the health action.
- Book: Chapter 4. Judging the Benefit of a Health Intervention. In: Know Your Chances: Understanding Health Statistics.
- Book: How likely are the benefits of treatment for me? How common are the harms? In: Smart Health Choices: Making Sense of Health Advice.
- Blog: Applying systematic reviews. Dr Chris Cates, EBM Website.
- Video: What’s the Risk? Understanding Absolute and Relative Risk Reduction. This is an NCCMT video lasting 8:24 minutes.