Read the following excerpts from the New York Times (December 17, 1996) article Chance of Heart Attack Increases for Those Who Suffer from Depression. A report in the December 16, 1996 issue of Circulation reports on a study that finds that those who are subjects to depression are four times more likely to have a heart attack than those who are not depressed.
The study involved 1551 people in the Baltimore area who took part in a study in 1980. In this study, interviewers asked questions like:
"In your lifetime, have you ever had two weeks or more in which you lost al interest and pleasure in things you usually cared about and enjoyed?"
They were also asked questions about unusual changes in their appetite and in their sleep. Based on this information, subjects were classified according to their depression status and their heart attack status was recorded.

Of the 444 who were classified as depressed, 27 had a history of heart attack.

1. Is this an observational study or is it an experiment? ·
A. experiment or B. observational
2. What is the response variable for this study?
A. heart attack status. or B. depression status
3. What is the explanatory variable for this study?
A. depression status or B. heart attack status
4. The value of 6 is a:
A. statistic or B. parameter

1551 persons were interviewed as part of the experiment and classified according to their answers in two groups "suffered depression" and "didn't suffer depression"

Afterward, the researchers looked up the medical history of all 1551 subjects and recorded how many of them had heard issues.

1.

An observational study is one where the researcher only determines the variables he wants to study without controlling or influencing any factor of the population of interest. Because there is no interference from the researchers, there is a high risk that confounding factors may affect the conclusions of it. These studies are usually precursors to experimental tests, it allows the researcher to have an idea of the association between the two variables but you cannot establish causal relationships from them.

This is an example of an observational study, the researcher merely recorded the variables of interest without interfering with any aspects that may influence them. (For example diet or exercise habits of the subjects)

2. and 3.

The objective of the study is to determine if "depression" causes "disposition to heart attacks"

The explanatory variable is "depression status of the subject" since it is what is suspected to influence over the heart condition of said subject and "heart attack status" is the response variable.

4.

Of the 1551 subjects originally studied, 444 were classified as depressed, of these 27 had heard attacks. You can calculate the "proportion of subjects that suffered depression and had heart attacks" as:

p': 27/444= 0.06 ⇒ 6%

Since this value was obtained from the sample, it is an estimation of the population parameter, also called statistic.

I hope it helps!