Fieldwork Design and Data Collection Reliability

Calculating Systematic Sample Intervals

Working out exactly where to stop along a transect requires a simple calculation. To find your systematic sampling interval, divide the total length of your transect by the number of samples you need to collect.

A student needs to take 8 sediment samples along a 120m stretch of coastline. Calculate the interval between each sample.

Step 1: Identify the total length and the number of samples required.

• Total length = $120\text{ m}$, Number of samples = $8$

Step 2: Substitute the values into the formula.

• $\text{Interval} = 120 \div 8$

Step 3: Calculate the final answer with units.

• $\text{Interval} = 15\text{ m}$

Sample Size and Representative Data

Why does asking just two people in a town centre give a weak conclusion? Taking a larger sample size makes your data a representative sample, meaning it accurately reflects the whole area.

• Small sample sizes lead to tentative, weak conclusions because they are easily skewed by anomalies (unexpected outlier results).
• By increasing the sample size, you reduce the impact of these anomalies and boost the reliability of your study.
• As a general rule in geography, a good sample size for physical features like pebbles is between 10 and 50 per site. For human geography, traffic counts should be conducted for set intervals (such as 5-minute windows) and repeated across multiple locations.

Accuracy vs Reliability

Hitting the bullseye on a dartboard is a great shot, but true skill is hitting it three times in a row. In fieldwork, accuracy is how close a measurement is to the true value, free from human or equipment error. You can improve accuracy by upgrading equipment, such as swapping a floating orange peel for a digital flow meter, or using calipers instead of a ruler to measure pebbles.

Reliability is about reproducibility and consistency. Data is reliable if someone else could repeat your exact method and get the same results. You can improve reliability by taking repeat measurements at the same site and calculating a mean to smooth out temporary fluctuations.

When collecting subjective data like an EQS, you can boost inter-observer reliability by working in pairs to agree on a consensus score, or by taking the mode of several students' opinions. Always follow standardised procedures, such as measuring river velocity at exactly 0.6 of the total depth every time.

Using Control Groups

To prove a new sea wall is stopping erosion, you need to know what the erosion looks like without one. In geography, a control group (or comparison site) is a baseline location where the independent variable you are studying is missing.

• Physical Geography: You might compare a managed beach that has wooden groynes against an unmanaged control beach to see the true impact on longshore drift.
• Human Geography: You could compare a newly regenerated high street with an un-regenerated control street to measure the difference in environmental quality.

Using a control group is essential because it proves that any changes you observe are actually caused by your specific variable, rather than just natural variation over time.