11.5 - 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.

The three types of correlation

1. Positive - As one variable increases, the other increases.
2. Negative - As one variable increases, the other decreases.
3. No correlation - There's no relationship between the two.

Why correlation isn't proof of cause

• A correlation doesn't by itself prove that one factor causes the other. It can arise for three reasons:
  1. Chance
  2. A third factor that affects both variables
  3. A genuine cause
• Example: ice cream sales and drownings both rise in summer, so they're correlated - but neither causes the other. A third factor, hot weather, increases 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:

1. Other variables are controlled - The investigation rules out other factors that could explain the change, so the link isn't down to something else.
2. A plausible mechanism - There's a scientific 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 sales and drowning meets neither, so it stays a correlation only.