A study was conducted on dietary anti-oxidant intake (Vitamin A and Vitamin E) and colon cancer. 120,000 people, aged 20-70 years, were selected at random from the total population of Melbourne in 1987 and invited to join the study. 110,700 of those invited agreed to participate. At the start of the study participants filled the baseline questionnaire including socio-demographic characteristics, height, weight, BMI (body mass index), dietary intake and other health risk factors (including smoking, exercise and stress). Every two years thereafter the authors re-sent the same questionnaire. At the end of the study, which was 1997 (10 years from baseline), researchers were still in contact with 64% of the study population and still collecting data from them. Outcome data (cancer episode and site) were available for 97% of the original study population from the Victorian Cancer Registry.

(a) What is the study design, the research question the main exposure and the main outcome ?

(b) What are the key features that led you to this choice .

(c) Can the same research question be studied in randomised controlled trial?