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Question $($I point$)$

A researcher finds $4000$ men over $40$ who exercise regularly, have not had a heart attack, and are willing to participate in the study. She assigns $2000$ of the men to a regular program of supervised exercise. The other $2000$ continue their usual habits. Both groups are followed for five years and fewer heart attacks are observed in the group assigned to a regular program of supervised exercise.

Reference: Ref $8-4$

The factor in the experiment is

a$)$ exercise program $($supervised or usual habits$).$

b$)$ use of a group continuing regular habits.

c$)$ whether a heart attack occurred.

d$)$ the length of the study.