Psychology / Unit 1: Introduction and Research Methods
1.5 Correlation
Notes
A correlation describes how two variables are related. It is measured by the correlation coefficient $r$, which ranges from $-1$ to $+1$.
- Positive: both rise together (height and shoe size). Negative: one rises as the other falls (hours of TV and test scores, in some studies).
- The closer $r$ is to $+1$ or $-1$, the stronger the relationship. Near 0 means little or no relationship.
- Correlation does not equal causation. A third variable may explain both. Ice cream sales and drownings rise together because of hot weather.
- Example: $r = -0.80$ is a strong negative correlation; $r = +0.20$ is a weak positive one.
Practice
Which correlation coefficient shows the strongest relationship?
Strength depends on distance from zero, not sign. $-0.90$ is closest to $-1$.
A study finds that students who eat breakfast have higher grades. A reporter says breakfast causes better grades. What is the best criticism?
Correlation alone does not prove cause. A third variable could explain it.
As the number of hours spent on a video game increases, sleep hours decrease. This relationship is:
Two variables move in opposite directions, which is a negative correlation.
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