Daily nitrate concentrations were recorded at a river monitoring station for 180 consecutive days. A researcher wishes to select 30 days for further investigation. The measurements tend to follow a cycle lasting about 6 days after releases from an upstream site.
Explain why selecting every sixth day could give a biased sample.
Describe how a simple random sample of 30 days could be selected.
State one reason why a second simple random sample of 30 days might lead to a different conclusion.
36 exam-style questions on WJEC A Level Maths 2.1 Statistical Sampling, covering 2.1.1 Statistical Sampling, 2.1.2 Statistical Sampling, 2.1.3 Statistical Sampling, 2.1.4 Statistical Sampling, and 2.1 Statistical Sampling. Each one has a worked solution and a mark scheme showing where the marks go.