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Scatter diagrams and correlation
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A scatter diagram (scattergram) shows the relationship between two co-variables in a correlation. Each dot is one participant.
Plotting a scatter diagram
Put one co-variable on the x-axis and the other on the y-axis, with labels and units.
For each participant, plot one point where their two scores meet. Do not join the dots.
Example: a participant who slept 7 hours and scored 15 on a memory test is plotted at (7, 15).
For each participant, plot one point where their two scores meet. Do not join the dots.
Example: a participant who slept 7 hours and scored 15 on a memory test is plotted at (7, 15).
Identifying the correlation
Positive correlation: the points go up from left to right (both increase). The sleep data above is positive.
Negative correlation: the points go down from left to right (one increases as the other decreases).
No correlation: the points are scattered randomly with no pattern.
The closer the points are to a straight line, the stronger the correlation. Points spread widely mean a weak correlation.
Negative correlation: the points go down from left to right (one increases as the other decreases).
No correlation: the points are scattered randomly with no pattern.
The closer the points are to a straight line, the stronger the correlation. Points spread widely mean a weak correlation.
Interpreting carefully
A scatter diagram can show a strong relationship, but not cause and effect.
You can describe the pattern ('as sleep increases, memory scores tend to increase') and point out any outliers (points far from the others).
Lines of best fit are not required, but describing the overall direction and strength is.
You can describe the pattern ('as sleep increases, memory scores tend to increase') and point out any outliers (points far from the others).
Lines of best fit are not required, but describing the overall direction and strength is.
On a scatter diagram, points go down from top left to bottom right and lie close to a line. Describe the correlation.
- Going down from left to right means negative.
- Points close to a line means strong.
Answer: A strong negative correlation.
Scatter diagrams plot two co-variables, one point per participant. Points rising left to right show a positive correlation; falling shows negative; no pattern shows none. Points close to a line mean a strong correlation. Scatter diagrams cannot show cause and effect.
The interactive lesson includes the diagrams for this topic.
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