Scatter plots and correlation
What this lesson covers
- Describe trend, association, strength and unusual points.
- r measures straight-line strength from −1 to 1.
- Association does not prove cause.
What to describe
- Trend: linear or curved.
- Association: positive or negative.
- Strength: how close the points are to the trend.
- Unusual features: outliers and groups.
Reading r
- r only measures straight-line relationships.
| r | Meaning |
|---|---|
| 0.95 | Strong positive |
| 0.5 | Moderate positive |
| 0 | No linear relationship |
| −0.8 | Strong negative |
Common mistakes
- Saying r = 0 means no relationship.It may be curved, so look at the graph.
- Saying a strong r proves cause.A third variable may explain both.
Your turn
- 1. What does r = −0.9 suggest?
- 2. Ice cream sales and sunburn rise together. Does one cause the other?
Show answer
1. A strong negative linear relationship 2. No, hot weather is a likely cause of both
What to take away
- Graph first, r second.
- Correlation is not cause.
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