Spearman's Rank interpretation

A student calculated Spearman's Rank correlation coefficient for the relationship between distance from a river source (km) and mean bedload size (mm) at 10 sites. Their result was rₛ = -0.78. The critical value at the 95% significance level for n = 10 is 0.564.

Analyse what this result tells the student about their hypothesis that bedload size decreases downstream. (5 marks)

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Mark scheme (5 marks) — Levelled:

• Level 3 (4–5): Identifies strong negative correlation; correctly states result exceeds the critical value (0.78 > 0.564) so is statistically significant at 95% — less than 5% probability of occurring by chance; supports the hypothesis (bedload decreases downstream); links to Bradshaw model / attrition / abrasion reducing particle size; cautious wording — correlation ≠ causation, anomalies possible.
• Level 2 (2–3): Identifies negative correlation and significance, but limited link to geographical theory.
• Level 1 (1): Basic identification of negative relationship only.

Indicative content: rₛ values: 0 = no correlation, ±1 = perfect; minus sign indicates as one variable rises, the other falls.