OCR A-Level Geography H481
Coastal Landscapes
Glaciated Landscapes
Dryland Landscapes
Earth’s Life Support Systems
Changing Spaces; Making Places
Trade in the Contemporary World
Global Migration
Human Rights
Power and Borders
Climate Change
Disease Dilemmas
Exploring Oceans
Future of Food
Hazardous Earth
Geographical Skills
Geographical Skills
Correlation, lines of best fit and Spearman's rank
- 3 things to know
- Components 01, 02 and 03
Overview
Recall questions on positive and negative correlation, correlation and causation, lines of best fit, residuals and extrapolation, Spearman's rank correlation coefficient, its formula and tied ranks, worked calculations for an outwash plain and an alluvial fan, null hypotheses, significance at the 0.05 level and critical values, and why small samples are unreliable.
What you need to know
Answer each one before you reveal it.
How are tied values ranked when calculating Spearman's rank?
What is a residual on a scatter graph?
Questions from this subtopic
Each one with the model answer underneath.
- 1
What is correlation?
Model answer
Correlation is the extent to which two variables change together, described by its direction, positive or negative, and its strength, from strong to weak.
- 2
Distinguish between positive and negative correlation, with a geographical example of each.
Model answer
In a positive correlation both variables rise together, such as wave height and fetch, whereas in a negative correlation one falls as the other rises, such as grain size and distance from the apex of an alluvial fan.
- 3
Why does a correlation not prove that one variable causes a change in the other?
Model answer
Both variables may be driven by a third factor or the link may be coincidence, so a cause has to be established by geographical explanation rather than by the statistic alone.
- 4
How should a line of best fit be drawn on a scatter graph?
Model answer
As a single straight line following the trend of the points, with roughly equal numbers of points on either side, ideally passing through the point plotted at the mean of both variables.
- 5
What is a residual on a scatter graph?
Model answer
A residual is the vertical distance between an observed point and the line of best fit, and a large residual marks an anomaly that needs explaining.
- 6
Why is it risky to extrapolate a line of best fit beyond the data?
Model answer
The relationship may change or break down outside the range that was measured, so a prediction made far beyond the observed values, such as future glacier retreat, can be badly wrong.
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