Question Constellation Sign up for free AQA · GCSE · Biology · Higher
Bias from thrown quadrats
Students estimated the population of limpets on a sea shore. The students were given a square quadrat.
Explain why throwing a quadrat is not a random method to estimate population size. Do not refer to safety in your answer.
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Marking points
- 1 Identifies a source of selection or reach bias in where the thrown quadrat lands.
- 2 Links that bias to an unrepresentative population estimate.
Full-mark answer
The students may choose or aim at particular locations, and the throw is limited by how far they can reach. The sampled sites can therefore be biased and not representative of the true limpet abundance.
Why this answer loses marks
Throwing is random because you do not know where the quadrat will land.
A throw can still be aimed and is limited by reach, so locations do not have an equal chance and may not represent the shore.