1. The Language of Studies
Lecture 8
Secs. 3.1 – 3.3
Fri, Sep 15, 2006

2. Observation vs. Experimentation
Observational study
Experiment
Response variable
Explanatory variable

3. Observational Study vs. Experiment
An observational study does not manipulate the explanatory variables.
An experiment does manipulate the explanatory variables.

4. Examples Which ones are observational and which are experiments?
Acute Consumption of Flavanol-Rich Cocoa and the Reversal of Endothelial Dysfunction in Smokers
Food buying habits of people who buy wine or beer: cross sectional study
Gory and erotic images can affect your vision

5. Example
Suppose researchers wish to determine whether there is a relationship between cocaine usage by pregnant women and birth defects.
Should this be an observational study or a designed experiment?
Why?

6. Example
A traffic engineer is studying a particular intersection to determine the traffic flow.
He needs to know the average number of cars that turn left, turn right, and go straight.
Should this be an observational study of a designed experiment?
Why?

7. Levels and Treatments The explanatory variable is typically denoted X.
The response variable is typically denoted Y.
Values of the explanatory variable are called levels.
If there is more than one explanatory variable, then combinations of their values are called treatments.

8. Confound It!
Confounding variable – A variable whose effect cannot be separated from the effect of the explanatory variables.
If a study has one or more confounding variables, then the researchers cannot attribute changes in the response variable to any one explanatory variable.
Normally, researchers try to control for all confounding variables (that they can think of).

9. Examples
In the following studies, what are the explanatory and response variables?
Can you think of any confounding variables?
Acute Consumption of Flavanol-Rich Cocoa and the Reversal of Endothelial Dysfunction in Smokers
Food buying habits of people who buy wine or beer: cross sectional study
Gory and erotic images can affect your vision

10. Do “Explanatory” Variables Really Explain?
A study cannot prove that variations in the explanatory variable really were the cause of variations in the response variable.
The study can only give evidence supporting that belief.
It may be the case that there is a third variable that is affecting both the explanatory and response variables.
It may be conceivable that the “response” variable affected the “explanatory” variable!

11. Evidence of Causation
The following are evidence (not proof) of causation.
The same association between the explanatory and response variables occurs in a variety of situations.
There is a plausible explanation of how the explanatory variable could affect the response variable.
There is no equally plausible third factor that could be affecting both the explanatory and the response variables.