1. Understanding Observational Studies
Lecture 12
Secs. 3.4
Wed, Feb 4, 2004

2. Types of Observational Studies
Retrospective study – The subjects are grouped by the response variable. Then the explanatory variable is measured for each group and compared.
Prospective study – The subjects are grouped by the explanatory variable. Then the response variable is measured for each group and compared.

3. Example
Suppose researchers want to study the relationship between the amount of fat in one’s diet and the incidence of heart disease.

4. Example
A retrospective study might begin by dividing the subjects into two groups.
Those currently with heart disease.
Those currently without heart disease.
Then they would investigate the subjects’ past eating habits.

5. Example
A prospective study might begin by dividing the subjects into two groups.
Those currently eating a low-fat diet.
Those currently eating a high-fat diet.
Then they would observe the subjects over the next several years to see which ones develop heart disease.

6. Retrospective Studies
In a retrospective study, the response variable’s value is often its present value.
The value of the explanatory variable is determined by looking at past records or through interviews of subjects.

7. Prospective Studies
In a prospective study, the explanatory variable’s value is often its present value.
The value of the response variable is determined by tracking the subjects over weeks, months, or years into the future.

8. Let’s Do It! Let’s do it! 3.4 – Left versus Right.
Major Principle in Statistics: The larger a sample is, the more likely it is to be representative of the population.
How large is the sample of left-handers?

9. Difficulties in Observational Studies
In an observational study,
The researchers cannot control the explanatory variables.
The researchers cannot control the confounding variables.
In the earlier example, is it plausible to believe that more of those on a high-fat diet would also smoke?

10. Assignment
Page 142: Exercises 6 – 11.
Page 167: Exercises 30 – 32.