Supplementary Exercise 3.4 of IPS7e
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A study on the association between alcohol consumption and survival after 
a heart attack. Note that the alcohol consumption refers to the period
before the heart attack.

It is an observational study (and not an experiment) because the alcohol 
consumption was not imposed as a treatment but chosen by the persons 
themselves and just recorded afterwards (presumably in connection with 
the heart attack). Also, the alcohol consumption was prior to the heart
attack, and hence the start of the follow-up period.

The response variable is survival after 4 years.

The explanatory variable is alcohol consumption prior to the heart
attack.

The individuals are people who were hospitalized after a severe heart
attack. It is therefore this subset of the population that the
conclusions of the study apply to. Of course one may speculate that any
findings would be valid for other people than those hospitalized after
a large heart attack, but this would indeed be speculation.