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.