Which of these describes a random sample?
The researcher asks the first 20 pupils who walk past the library.
Every pupil's name is numbered and 20 numbers are drawn from a hat.
Pupils who reply to a poster on the form room door take part.
The researcher picks out 20 pupils who look as if they would be willing to help.
182 exam-style questions on OCR GCSE Psychology 7.1 Planning Research, covering 7.1.1 Hypotheses, 7.1.2 Variables, 7.1.3 Experimental Designs, 7.1.4 Populations and Sampling, 7.1.5 Ethical issues, and 7.1.6 Dealing with ethical issues and BPS Code. Each one has a worked solution and a mark scheme showing where the marks go.