8.5 - Correlation & Cause
Correlation, and why it is not cause
- A correlation is a relationship between two variables, where a change in one is linked to a change in the other.
- There are three types:
- Positive - As one variable increases, the other increases.
- Negative - As one variable increases, the other decreases.
- No correlation - There is no relationship between the two.
A correlation does not prove cause
- A correlation on its own does not prove that one thing causes the other. It can arise for three reasons:
- Chance - The link is a coincidence.
- A third factor - Something else affects both variables.
- A genuine cause - One variable really does cause the change in the other.
- For example, ice cream sales and drownings both rise in the summer, so they're correlated - but neither causes the other. A third factor, hot weather, raises both.
Showing that one factor causes another
- Showing that one factor causes a change in another needs more than a correlation.
- Two conditions have to be met:
- Other variables are controlled - The investigation rules out other factors that could explain the change.
- A plausible mechanism - There's a clear explanation for how one factor brings about the change in the other.
Applying the two conditions
- Smoking and lung cancer - Meets both: controlled studies ruled out other factors, and there's a known mechanism by which chemicals in tobacco smoke damage lung cells.
- Ice cream and drowning - Meets neither, so it stays a correlation only.
TermDefinition
CorrelationA relationship between two variables where a change in one is linked to a change in the other.
Positive correlationA relationship where as one variable increases, the other also increases.
Negative correlationA relationship where as one variable increases, the other decreases.
Key points to remember
How were these notes?