QUESTION 34 (5 marks)
A saltmarsh reserve is 60% low marsh and 40% upper marsh. The habitats differ in flooding frequency and vegetation. Twenty quadrats can be sampled.
a) Describe a stratified random sampling plan using proportional allocation. [3 marks]
b) Explain why placing all quadrats beside an accessible boardwalk would weaken the inference about the entire reserve. [2 marks]
Practice marking scheme
Answer
a) Define/map the two strata; randomly place 12 quadrats in low marsh and 8 in upper marsh. b) Boardwalk sampling may overrepresent one microhabitat and is unrepresentative.
Working
The two habitats form strata. Allocate 20 × 0.60 = 12 quadrats to low marsh and 20 × 0.40 = 8 to upper marsh, selecting positions randomly within each stratum. Use consistent quadrat size and counting rules. Convenience sampling beside the boardwalk may omit less accessible conditions, so observed composition may not represent the whole reserve.
Marking criteria
- Identifies and maps the two habitat strata. [1 mark]
- Allocates 12 and 8 quadrats respectively. [1 mark]
- Uses random positions within both strata. [1 mark]
- Identifies convenience sampling as potentially unrepresentative or biased. [1 mark]
- Links this to missing or overrepresenting particular microhabitats. [1 mark]
Practice question aligned to the current QCAA syllabus; review the worked solution and marking criteria.
View the QCAA syllabusCompare your working with the guide above.