Links Between Factors, Correlation and Causation

Risk factors for CVD rarely act alone. A person who smokes and has high blood pressure has a much greater risk than someone with only one of these factors, because the effects combine. Many factors are also linked to each other: for example, a poor diet and lack of exercise both lead to obesity, which in turn raises blood pressure.Studies of risk factors look for a correlation: a relationship in which two variables change together. In a positive correlation both increase together; in a negative correlation one increases as the other decreases.A correlation does not prove causation, which means that a change in one variable directly brings about a change in the other. A correlation may arise by chance, or because a third factor affects both variables. This is called a confounding variable.Example: people who carry a cigarette lighter are more likely to develop heart disease. The lighter does not cause the disease; both are linked to smoking.To support a causal link, scientists look for:1. a large, representative sample so the result is unlikely to be due to chance
2. control of other variables, or matching groups for age, sex and other risk factors
3. a plausible biological mechanism that explains how the factor causes the disease
4. repeated studies by other groups that give the same result.