Scatter graphs
- A scatter graph plots pairs of values (bivariate data) to see if two variables are related.
Correlation
- Positive correlation: as one variable increases, so does the other. Negative correlation: as one increases, the other decreases. No correlation: there is no pattern.
- Strong correlation means the points lie close to a straight line. Weak correlation means they are more spread out.
- Correlation does not prove that one thing causes the other. Ice-cream sales and sunburn cases both rise in hot weather.
Lines of best fit
- Draw one straight line through the middle of the points, following the trend, with about the same number of points on each side.
- Use it to estimate values. Estimating within the range of the data (interpolation) is fairly reliable. Estimating outside it (extrapolation) is unreliable, because the pattern may not continue.
Outliers
- An outlier is a point that doesn't fit the pattern. Ignore it when drawing the line of best fit.
Key terms
- Scatter graph
- A graph of pairs of values, used to look for a relationship.
- Bivariate data
- Data with two variables for each item, such as height and shoe size.
- Positive correlation
- As one variable increases, the other increases.
- Negative correlation
- As one variable increases, the other decreases.
- Line of best fit
- A straight line that follows the trend of the points.
- Outlier
- A point that doesn't fit the pattern.
- Interpolation
- Estimating a value within the range of the data.
- Extrapolation
- Estimating a value outside the range of the data.