A researcher collects samples of tap water in four towns within ten counties, totalling 40 towns. The samples are taken to a laboratory where they are tested for the presence of toxins.

A researcher randomises five different doses of fertilizer to 30 plants in a greenhouse (i.e. each dose is applied to six plants). The height of each plant is observed weekly for the next five weeks.

A researcher collects blood samples from 10 smokers and 10 non-smokers every week for a period of five weeks, to assess their blood cholesterol levels.

A researcher selects five schools at random within a county, three classrooms within each school and 10 students within each classroom. Then, the students’ height and weight are measured yearly for a period of three years.