Analysing Your Results: Does the Data Support the Hypothesis?

📈 Analysing Your Results: Does the Data Support the Hypothesis?

Once your data is plotted, you need to analyse it critically. This means more than just saying "the results support the Bradshaw Model." You need to describe the pattern, reference specific data values, identify anomalies, and offer geographical explanations.

Use the TACT Framework to describe patterns

Spearman's Rank Correlation Coefficient

Spearman's rank is a statistical test that measures the strength of the relationship between two variables. It is used in fieldwork to test whether the pattern you see in a scatter graph is statistically significant — i.e., whether it is likely to reflect a real relationship rather than just random variation.

The result is a correlation coefficient, written as r_s, with values ranging from:

• +1.0: perfect positive correlation (as one variable increases, so does the other, exactly)
• 0: no correlation
• -1.0: perfect negative correlation (as one increases, the other decreases, exactly)

To determine whether your result is statistically significant (i.e., real rather than due to chance), compare it to a critical value table. At the 95% confidence level with 14 pairs of data, the critical value is ±0.536. If your r_s exceeds this threshold (in either direction), you can be 95% confident that the correlation is real.

For example: if r_s = +0.71 for velocity vs distance downstream, this exceeds the critical value of 0.536, so you can conclude that there is a statistically significant positive correlation between distance downstream and velocity — supporting the Bradshaw Model.

Explaining Anomalies

An anomaly is a data point that does not fit the overall pattern. You must never simply ignore an anomaly — you must explain it using geographical reasoning. Common causes in river fieldwork include:

• A tributary joining just upstream of your site, which spikes discharge and velocity unexpectedly
• A waterfall or rapid immediately upstream, which pools water and reduces velocity above it
• A large boulder or obstruction mid-channel creating local turbulence
• A narrowing of the channel (such as a gorge or rock band) that speeds up the flow at one site
• Human interference: a weir, a culvert, or an abstraction point for water supply
• Human error on the day: the orange got caught on vegetation; the tape measure was not held taut; depth readings were taken in an unrepresentative part of the channel

Always consider both geographical causes and human error. In the exam, saying "the anomaly at Site 4 may have been caused by a tributary joining 30 m upstream, adding discharge and temporarily increasing velocity before it equalised" scores far more than "there was an anomaly that did not fit the trend."