GCSE · Geography · AQA · Spec 8035

Selecting and recording fieldwork data

Imagine measuring every rock on a beach. Every single one. You'd still be there next summer. So how do geographers get a trustworthy answer without measuring everything?

Sampling strategies

Measure every rock on the beach? Impossible.

Each card is one (made-up) geographer's way of choosing a sample. Pick a card, then pick the strategy you think they used.

Still to sort

Systematic (0)

A predefined system decides what goes in the sample.

Where the line is: The rule is fixed before you start (every 10th, every 5 metres), so it isn't random, even though you don't pick each one yourself.

Random (0)

You have no control over which data ends up in the sample.

Where the line is: No rule and no choosing: chance decides. If you pick what catches your eye, you're in control, so it isn't random.

Stratified (0)

Pre-existing knowledge of the area shapes the sample to make it more representative.

Where the line is: It starts from something you already KNOW about the place, like what it's made of, and builds the sample to match.

7 of 7 still to sort.

You'd still be there next summer. So geographers measure a sample: a small selection of data that can represent the whole, much bigger population. The only question is how you choose it, and there are three main ways.

Choosing your sample

Which plan would you back?

A (made-up) class wants to find out the overall shape of a beach and the size of its pebbles. They know the three strategies, and now they're arguing about which one to use, how many pebbles to measure, and where to measure them.

Which of these is closest to what you think right now?

Random sampling is always the best, because it's the fairest.The bigger the sample the better, so measure as many pebbles as you possibly can.Three pebbles is plenty. It's quick, and a pebble is a pebble.Just do one transect, through the part of the beach that looks most unusual. That's the interesting bit.There's no single right answer. It depends on the beach, what we want to find out, and how much time we've got.

How sure are you?

I'm sureI think soI'm guessing

Random sampling is always the best, because it's the fairest.

This is a common misconception

It's a very natural thought, but there's no league table of sampling strategies. None of them is 'the best' in general. The right choice depends on where you're collecting data, how big the whole population of data is, and what conclusions you're hoping to reach. Random suits some enquiries; systematic or stratified suit others.

The bigger the sample the better, so measure as many pebbles as you possibly can.

This is a common misconception

A bigger sample does help you reach meaningful conclusions, but only up to a point. The sample also has to be small enough to collect in the time and with the resources you actually have. So the right size is a balance, and it varies with the location, the time available and what the enquiry is about.

Three pebbles is plenty. It's quick, and a pebble is a pebble.

This is a common misconception

Quick, yes, but three pebbles is far too few to form meaningful conclusions about a whole beach. Manageable matters, but it isn't the only thing that matters. The sample needs to be big enough to tell you something.

Just do one transect, through the part of the beach that looks most unusual. That's the interesting bit.

This is a common misconception

Tempting, but this plan has two problems. A transect is an imaginary line between two points, along which data is collected, and one line across a whole beach isn't a large enough sample to show the beach's overall shape. Worse, choosing a part BECAUSE it looks unusual means the sample isn't representative: the measurements are biased. Bias means favouring some data over others, and it can make your conclusions meaningless, because the data no longer shows the whole picture. Avoiding it is one of the main reasons for having a clear sampling strategy. Sampling a series of transects, or splitting the area into a grid and choosing some squares with a random number generator, keeps your own preferences out of it.

There's no single right answer. It depends on the beach, what we want to find out, and how much time we've got.

This matches the evidence

Exactly. The best strategy depends on the location, the size of the population of data and the conclusions you want to reach. The sample size should be large enough for meaningful conclusions but manageable in the time and resources available. Good geographers choose, and then say why.

No strategy is best in general, and 'as many as possible' isn't a sample size: choose what fits the place and the question. And don't pick a spot because it looks unusual. That gives biased data, and bias can make your conclusions meaningless.

Primary or secondary?

Standing at the river doesn't make it primary

Now zoom out from how you sample to what you collect. You're at a river field site. For each piece of data, tick the sets it belongs in: one, both or neither.

A Describes this river field site
B Primary data: collected in this fieldwork

The width of the channel, measured with a tape measure
The depth of the channel, measured with a ruler
The gradient of the channel, measured at the site
The height of the river above sea level, read from a map
The distance of the site from the river's source, worked out from another source of information
Answers to a questionnaire you give to people in the nearby town on your fieldwork day
Survey results another school collected in a different town last year

Measuring techniques

What's in the fieldwork kit bag?

Showing 3 layers: Field site, Shape of the land, Characteristics and vegetation

Layers

Explore

tap each piece of kit

Characteristics of the land, such as how easily water infiltrates or whether there's pollution, plus a quadrat for sampling vegetation.

Switch the two layers on and off, then tap each piece of kit to see what it measures and how.

Predict, then check

You've got the kit. Now the real question: what do you actually point it at?

Your enquiry question is: 'How does the depth of the river change from its source to its mouth?' What data do you need?

Your turn

Plan it, justify it, then mark it

Illustrative enquiry question: 'Does pebble size change along a 1 km beach?' Describe and justify how you would collect data to answer this question. Include where you would sample, which sampling strategy you would use, and how big your sample would be. [4 marks]

Your answer

0 words · your answer stays on this page and is not sent anywhere.

The mark scheme

Where: samples at a number of sites along the beach (for example, a series of transects spread along it) so that the sites can be compared, because the question is about change along the beach. (1 mark)
Strategy: names a sampling strategy and describes it in use, for example systematic sampling by measuring every 10th pebble inside a quadrat at each site. (1 mark)
Size: justifies the sample size as large enough to form meaningful conclusions but manageable in the time and resources available. (1 mark)
Bias: explains that following a clear strategy (not picking sites or pebbles because they look unusual) avoids bias, so the data shows the whole picture. (1 mark)

Model answer

I'd measure pebbles at several sites spread along the beach, using a series of transects, because the question is about how pebble size changes along the beach, and that only shows up if I can compare sites. At each site I'd use systematic sampling: put down a quadrat and measure every 10th pebble inside it. I'd measure enough pebbles at each site to reach meaningful conclusions, but not so many that I couldn't finish in the time I had. Following a fixed system also stops me choosing pebbles or spots because they look unusual, which would bias the data so it didn't show the whole picture.

Where, which strategy, how many, and how you'll avoid bias. Justify each choice by linking it back to the enquiry question.

WHAT YOU'VE LEARNED

A quick recap of today's lesson.

Which data to collect, what to measure it with, and how to pick a sample that shows the whole picture rather than a biased one.

What you need to know

Tell primary data from secondary data, including data that describes your site but was collected by someone else
Match measuring techniques to what they measure: tape measure or ruler, clinometer, quadrat, infiltrometer, particle capture card and colour-change strips
Recognise systematic, random and stratified sampling from a description
Choose a sample size and explain why it is big enough but still manageable
Describe and justify a data collection method, including where you'll sample and how you'll avoid bias

The big picture

Good fieldwork starts with choosing the right data. Primary data is collected in your own fieldwork; secondary data was collected by someone else, even when it's about your site. Each technique has its job: a tape measure or ruler for the shape of the land, a clinometer for tall features, and a quadrat, infiltrometer, capture card or test strip for vegetation, infiltration and pollution. You can't measure everything, so you sample systematically, randomly or by stratifying, with a size and location that suit the enquiry. The aim is unbiased, comparable data that helps answer the enquiry question.

Key points

1Primary data is collected as part of your own fieldwork. Secondary data was collected by someone else, usually beforehand. Data about your site, such as the river's height above sea level from a map, can still be secondary.

2Every piece of data you collect should help answer the enquiry question.

3Shape of the land (height, depth, width, length, gradient): usually a tape measure or ruler. For a cliff or tree, measure the distance to the base, the height from the ground to your eye, and the angle from eye level to the top with a clinometer.

4Characteristics of the land: an infiltrometer times how long a set amount of water takes to infiltrate; sticky particle capture cards compare air pollution; colour-change strips show chemicals in water (darker = more). A quadrat samples vegetation.

5A sample is a small selection that can represent a much bigger population. Systematic = a predefined system. Random = no control over what gets in. Stratified = using existing knowledge to make the sample more representative.

6No strategy is best in general: it depends on the location, the size of the population and the conclusions you want. Sample size must be large enough for meaningful conclusions but manageable.

7Plan where you sample (a series of transects, or grid squares chosen with a random number generator) and avoid bias. Biased data doesn't show the whole picture and can make conclusions meaningless.

8Measurements gain value through comparison: several sites, similar places or earlier data, and paired measurements.

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Worked example

Problem

Illustrative enquiry question: 'How does vegetation cover change with distance from a footpath?' Plan the data collection. Then work out the vegetation cover for one quadrat: a grid quadrat of 25 squares has vegetation in 18 of them.

⚠ Watch out

Thinking 'random' means 'whatever I fancy'. If you pick the rocks that catch your eye, YOU are controlling what gets into the sample. That isn't random sampling, it's bias.

🧠

Memory hook

Rule, Luck, Knowledge. Systematic follows a RULE you set in advance. Random leaves it to LUCK, because you have no control over what gets in. Stratified uses what you already KNOW about the place to shape the sample.

✓

Check yourself

Cover the page. Name the three sampling strategies and make up your own example of each. Then decide: is the distance of your river site from the source primary or secondary data, and why?

Flashcards

(14)

What is primary data?

Data collected as part of a fieldwork exercise, by you.

What is secondary data?

Data that has been collected by someone else, usually beforehand.

Which three measurements does the clinometer method use to find the height of a tree or cliff?

The distance to the base, the height from the ground to your eye, and the angle between eye level and the top (measured with the clinometer).

What does an infiltrometer measure, and how is it used?

How easily water infiltrates the ground. Hammer it in firmly, pour in a set amount of water, and record how long it takes to infiltrate.

How does a particle capture card measure air pollution?

Airborne particles stick to its sticky surface. Cards hung in different areas for a set amount of time are compared.

On a water-testing strip, what does a darker colour mean?

More of the chemical, such as phosphates or nitrates, is present in the sample.

Why do geographers take a sample?

It isn't possible to measure every bit of data, so they take a small selection that can represent the whole population.

What is systematic sampling?

Using a predefined system to choose the sample, e.g. measuring every 10th rock in a quadrat.

What is random sampling?

Having no control over which data gets into the sample, e.g. picking out 15 rocks with your eyes closed.

What is stratified sampling?

Using pre-existing knowledge to make the sample more representative, e.g. 50% granite and 50% limestone in an area made of both.

What is bias, and why does it matter?

Favouring some data over others. It can make conclusions meaningless, because the data doesn't show the whole picture.

Which sampling strategy is the best one?

None in general. It depends on the location, the size of the population of data and the conclusions you want to reach.

How big should a sample be?

Big enough to form meaningful conclusions, but small enough to collect in the time and with the resources available.

What is a transect?

An imaginary line between two points, along which data is collected.

Tap any card to flip it, or use Study as deck to go through them one at a time. In the full lesson these run as a spaced-repetition deck — you rate each card Hard, Good or Easy and the tricky ones keep coming back until they stick.

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Measure every rock on the beach? Impossible.

Which plan would you back?

Standing at the river doesn't make it primary

Plan it, justify it, then mark it

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