Scatter Plots
What is a Scatter Plot?
A scatter plot (scatter diagram) displays two variables for the same set of data, with each variable on one axis. Each observation becomes a point on the graph.
Use scatter plots to look for relationships (correlation) between variables.
Types of Correlation
Positive correlation: as x increases, y tends to increase β points slope from bottom-left to top-right
- Example: height and shoe size
Negative correlation: as x increases, y tends to decrease β points slope from top-left to bottom-right
- Example: altitude and temperature
No correlation: no clear pattern β points scattered randomly
Strength of Correlation
Strong: points cluster closely around a line Weak: points are more spread out but show a trend
Line of Best Fit
A line of best fit (trend line) summarises the trend. It:
- Passes through the mean point (xΜ, Θ³)
- Has roughly equal numbers of points on each side
Use it to make predictions (interpolation within the data range is more reliable than extrapolation outside it).
Correlation β Causation
Just because two variables are correlated doesn't mean one causes the other. Both may be influenced by a third variable (confounding factor).
Example: ice cream sales and drowning rates are positively correlated β both are caused by hot weather, not each other.