Q7

Questions 5 - 7 refer to the information below about the British Doctors’ Study. The British Doctors’ Study was an epidemiological cohort study which started in 1951 and ran until 2001. It set out to establish whether, as strongly suspected, there was a link between smoking and health outcomes. It was the first study to prove the link between smoking and lung cancer and was hugely influential in informing government moves to reduce smoking. A very large number of male doctors were sent health surveys at regular intervals. These asked questions about any health issues they might have as well as about lifestyle factors such as alcohol consumption, diet and smoking habits. The simplified graph below illustrates one of their findings.

What was the independent variable in this study?
(A) age
(B) smoking habits
(C) gender
(D) cancer rate