An amusing article in the British Medical Journal once noted that there was no rigorous statistical evidence to support parachute use as preventing injury when jumping from planes due to the absence of any randomised-controlled-trials.

a)[2 marks]Why is it difficult to assess cause and effect relationships based on observational studies?

Disregarding any ethical issues answer the questions below.

b)[3 marks]How should participants be chosen for the study and how should they be allocated to the control or treatment group?

c) [2 marks] How should the control group be treated compared to the treatment group?

d) [1 mark] What would it mean for the experiment to be double-blind in this case?