You are interested in finding out whether middle-aged men who have premature heartbeats are at greater risk of developing a myocardial infarction (heart attack) than men whose heartbeats are regular. Electrocardiogram (ECG) examinations are performed on all male office employees 35 years of age or older who work for oil companies in Houston. The ECG tracings are classified as irregular or regular. Five years later, myocardial infarction rates are compared between those with and those without baseline ECG irregularities. What kind of study is this? a. Cross-sectional study
b. Case-control study
c. Prospective cohort study
d. Retrospective cohort study
e. Clinical trial
f. Community trial