Correlation is a relationship between two variables, and causation occurs when one of those variables has an effect on the other. A common mistake reporters make when writing about medical studies is confusing correlation and causation. Two variables in a study can be related without one actually being directly caused by the other.
Deeper dive
For example, many people who suffer from alcohol use disorder also smoke cigarettes. Alcohol addiction and tobacco addiction are correlated, but one doesn’t cause the other. In a study that compares drinkers and non-drinkers, heavy drinkers would have higher rates of pancreatic cancer than non-drinkers. But it’s impossible to know if the cancer was caused by their drinking or by something else that made them different from the nondrinkers, such as higher rates of smoking.
Here’s another example of how correlation can cloud the interpretation of a study. The amount of sodium a person gets in their diet is closely correlated to the total calories they eat. In other words, the more a person eats, the more sodium they’re likely to take in. Eating a lot of calories often also leads to obesity. Both obesity and high-sodium diets are believed to contribute to high blood pressure. So what’s the primary driver of high blood pressure in a scenario like this: sodium or obesity? Those are the kinds of questions researchers try to disentangle in their studies.
Observational studies can only show correlation. They can’t show causation. When covering observational studies, it’s important to use language that makes the limits of the research clear.
Seasoned health reporters will eschew wording in their leads or headlines that reads like this: “A new study shows that short sleep may cause weight gain.”
Instead, they aim for wording that suggests a less direct relationship: “A new study shows that people who don’t get at least seven hours of sleep a night are more likely to gain weight compared to those who snooze less.”
That’s the most accurate way of describing the comparison that’s being made in the study, but it can also be a little wordy. Here’s another way that would work if you’re tight on space: “A new study shows short sleep is linked to weight gain.”