International A Level Biology / Cardiovascular health and risk
Risk, correlation and cause
Edexcel International A Level Biology (YBI11), chapter 3: Cardiovascular health and risk, specification point 1.15, 1.17. Free lesson.
Read the revision notes · International A Level Biology past papers
Risk measures the chance of harm in a group over a stated time, while relative risk compares that chance between groups. Understanding the difference between correlation and cause helps you judge health evidence and avoid being misled by how risky something feels.
What risk means
In biology, risk is the probability that something harmful, such as developing or dying from a disease, happens over a stated time. A complete risk gives the chance, the group and the time. This makes it possible to compare like with like. Risk is often expressed per 100 000 people per year. For example, 30 per 100 000 per year is a risk. The number 30 on its own is not enough to compare groups.
How to calculate risk and relative risk
If groups are different sizes, comparing their numbers of deaths directly is unfair. Scale each group to the same size, then compare the risks. The relative risk is the risk in the exposed group divided by the risk in the unexposed group. A relative risk of 3 means the exposed group was three times as likely to experience the outcome.
How correlation differs from causation
Correlation means two variables change together. It describes a trend in data, such as one shown on a scatter graph. Causation means that a change in one variable brings about a change in the other. To support a cause, you need a biological mechanism and evidence from controlled studies. A correlation could happen by chance or because a third variable is linked to both. A positive correlation means that as one variable increases, the other increases too. It does not show that one causes the other.
How a third variable can explain a link
People who carry a lighter may have more heart attacks, but that correlation does not mean lighters cause heart attacks. Smoking is linked to both carrying a lighter and heart disease, so it can explain the apparent link. Smoking has a mechanism: chemicals in smoke damage the endothelium, the artery lining, which starts atherosclerosis. A lighter has no such mechanism. A third variable linked to both is called a confounding variable. To control for smoking, compare smokers who carry lighters with smokers who do not. You would expect no difference in heart attack risk. Studies can disagree…
Why perceived risk differs from actual risk
Perceived risk is what people think the risk is. Actual risk is what the data show. People often overestimate rare, dramatic events, such as a plane crash, and underestimate everyday habits, such as a diet high in saturated fat or inactivity. Rare events can feel risky because they are vivid, widely reported and seem outside our control. Everyday habits feel familiar and chosen. Their harm to arteries builds slowly over years, so people may not change habits that matter for heart disease. When comparing perceived and actual risk, state where they are close as well as where they differ.