5 original questions · Application and evidence
Use the context and data to draft a response, then compare your reasoning with an explanatory example.
Eight non-overlapping 0.50 m² quadrats were placed at randomly selected positions across a 2000 m² field. A plant count and soil-moisture reading were recorded at each position. The field boundary defines the sampling area.
| Quadrat | Plant count | Soil moisture / % |
|---|---|---|
| 1 | 2 | 20 |
| 2 | 0 | 15 |
| 3 | 5 | 35 |
| 4 | 1 | 22 |
| 5 | 4 | 32 |
| 6 | 0 | 18 |
| 7 | 3 | 30 |
| 8 | 1 | 25 |
Original illustrative data. Use the stated assumptions and units; these are not examination-board questions.
This worksheet includes questions, data, explanatory responses and self-check points.
These are original revision check points, not official examination marks.
Total count = 16, so mean count = 16/8 = 2 plants per quadrat. Density = 2/0.50 = 4 plants m⁻². Estimated field total = 4 × 2000 = 8000 plants, assuming the sample adequately represents the defined field.
Common confusion: Multiplying the per-quadrat mean directly by field area ignores the quadrat’s area.
Higher moisture readings are generally associated with larger counts: the 15% and 18% quadrats have no plants, while those at 32% and 35% contain four and five. The pattern is not perfectly proportional; for example, the counts at 22% and 25% are both one.
Common confusion: A description of association should not automatically assert that moisture caused the differences.
The observations show an association, but moisture was not independently manipulated. Light, nutrients or disturbance could covary with moisture and affect abundance. A controlled comparison varying moisture while making other relevant factors comparable would better test its causal effect. The field table alone does not predict the size of that effect.
Common confusion: Naming a statistical association or collecting more observational quadrats does not remove every confounding factor.
Random placement gives field positions a defined chance of inclusion and reduces deliberate choice of plant-rich patches. Enough independent samples are needed to assess spatial variation. A slope pattern instead calls for position-based sampling along a justified transect, measuring both abundance and relevant environmental conditions.
Common confusion: The sampling design should match the question; a transect and a whole-field random sample answer different questions.
Increase the number of independently placed random quadrats and report the distribution of counts or a justified uncertainty estimate. This better assesses spatial variation and the precision of the estimated mean. It does not make every field location identical or turn 8000 into an exact census.
Common confusion: Repeatedly recounting the same quadrat is not equivalent to adding new independent locations.