A recent study claimed that going to the gym reduces your risk of cancer. The study looked at thousands of middle-aged adults and saw that those who frequently exercised (at least 30 minutes 3 times per week) were significantly less likely to develop cancer in the next 10 years. Which of the following factors is a potential confounding factor?
a) The average age of all adults in the study was 46, while the average age of all Americans is 53.
b) The study included 8000 adults who frequently exercised and 6000 adults who did not frequently exercise.
c) The average income of adults who frequently exercised was $78132, and the average income of adults who did not frequently exercise was $61924.
d) 2000 out of 8000 adults who frequently exercised smoke regularly, and 1500 out of 6000 adults who do not frequently exercise smoke regularly.