Choose sampling and field-data methods
Select quadrats, transects, traps, water tests or surveys to match the organism, habitat and environmental question.
Exam scope and assessment notes
Original StudyForA teaching and practice, not Cambridge-authored questions, mark schemes or official grades. Cambridge expects candidates to study all eight topics, carry out investigative work and learn teacher-selected case studies. This course does not replace a centre's named case-study examples, current local environmental data, supervised fieldwork or the candidate's own investigation. Practice datasets are illustrative, not observations from a real place. No official assessment or certification is provided. The examples and practice datasets in these lessons are fictional and formative; learners must add their centre's verified case studies and current local evidence.
Before you begin
- Cambridge recommends prior environmental/science study equivalent to IGCSE or O Level. This is an AS-only qualification; use the exam-year syllabus and confirm centre entry details.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What makes environmental evidence useful?
What you will learn
- Select an appropriate field method and sampling frame
- Distinguish random, systematic and stratified sampling
- Explain representativeness, repeatability, safety and method limitations
Match method to the target
Quadrats estimate abundance or percentage cover for stationary organisms. Transects measure change across a gradient. Sweep nets, beating trays, kick sampling, light traps and capture–mark–recapture target different organisms or contexts.
Water turbidity and other field indicators require a specified instrument, unit, location and sampling depth. Questionnaires and interviews measure reported experience, not direct environmental conditions.
Representativeness is a design choice
Random sampling can reduce selection bias; systematic spacing can reveal a gradient. Choose between them using the study's size, ease of access, knowledge of the environment, precision, likely bias and efficiency. Stratification can be a useful extension when important habitat zones must each be represented.
The syllabus techniques include open-frame, grid and point quadrats, pitfall traps, sweep nets, beating trays, kick sampling, light traps and capture–mark–recapture; questionnaires, interviews and water turbidity are also listed. Match each method to the organism or variable and state its limitations.
The Lincoln index estimates population size: N = (n1 × n2) ÷ m2, where n1 is the first capture, n2 is the second sample and m2 is marked recaptures. It assumes a closed population, mixing, retained marks and equal catchability; check those assumptions before interpreting the estimate. Fieldwork also needs permissions, risk assessment and minimal habitat disturbance.
Sources for this lesson
Pause and explain
Which design is suited to measuring change along a habitat gradient?
Worked example
Measure how plant cover changes from a footpath edge into woodland.
Show the worked solution
- Lay out several transects across the gradient.
- Place equal-sized quadrats at fixed intervals and use one cover-estimation rule.
- Repeat at comparable locations and record shade and disturbance.
Answer A systematic transect with quadrats at fixed intervals can show how cover changes from the edge toward woodland. Repeating transects helps reveal spatial variation. Record shade, soil and disturbance because distance from the path may not be the only influence.
Try a topic test
Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.
Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
A meadow has slow-growing plants clustered in patches. How should you estimate the target species' abundance, percentage cover and frequency?
Give me a hint
Use repeated quadrats and keep the three measurements distinct.
Compare my reasoning
- Identify the relevant data, place or process in the prompt.
- Apply select an appropriate field method and sampling frame to the stated context.
- Explain the inference and one limitation or trade-off.
Define the meadow boundary and choose random quadrat locations from a sampling frame; stratify if distinct habitat zones must be represented. Count individuals or use a stated ACFOR scale for abundance. Estimate percentage cover as covered area divided by quadrat area × 100, and frequency as quadrats containing the species divided by total quadrats × 100. Repeat the sample and report the method and uncertainty.
Look for these in your work
- Select an appropriate field method and sampling frame
- Uses relevant evidence and explains the environmental mechanism
- Distinguishes observation, inference and limitation
A capture–mark–recapture sample catches 40 fish, marks them, then catches 50 fish of which 10 are marked. Estimate the population and name an assumption.
Give me a hint
Use N ≈ (first catch × second catch) ÷ marked recaptures.
Compare my reasoning
- Identify the relevant data, place or process in the prompt.
- Apply distinguish random, systematic and stratified sampling to the stated context.
- Explain the inference and one limitation or trade-off.
N ≈ (40×50)/10 = 200 fish. This estimate assumes marked fish have mixed back into the population and are as likely to be recaptured as unmarked fish; migration, deaths or lost marks can bias the estimate.
Look for these in your work
- Distinguish random, systematic and stratified sampling
- Uses relevant evidence and explains the environmental mechanism
- Distinguishes observation, inference and limitation
A student samples only easy-to-reach stream banks and calls the result representative. Improve the sampling design.
Give me a hint
Use a defined sampling frame and account for bank conditions.
Compare my reasoning
- Identify the relevant data, place or process in the prompt.
- Apply explain representativeness, repeatability, safety and method limitations to the stated context.
- Explain the inference and one limitation or trade-off.
Define the stream reach and select random points from a map or stratify by relevant features such as shade and flow. Sample consistently within each group and record inaccessible sites. Convenience sampling may over-represent easy-to-reach conditions.
Look for these in your work
- Explain representativeness, repeatability, safety and method limitations
- Uses relevant evidence and explains the environmental mechanism
- Distinguishes observation, inference and limitation
Common mistakes
- Using quadrats for mobile animals without an appropriate adaptation
- Assuming systematic spacing guarantees a representative sample
- Reporting a precise estimate without testing the method assumptions
What can you explain now?
Which design is suited to measuring change along a habitat gradient?
Compare with the explanation
A transect with consistently placed sample points
Following the gradient allows comparable observations at different positions.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
What makes a sample representative of the population or habitat being studied?
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.