1. The Language of Studies
Lecture 8
Secs. 3.1 – 3.3
Fri, Sep 10, 2004

2. Observation vs. Experimentation
Observational study – A study in which the subjects are not manipulated or controlled in any way, but simply observed.
Designed experiment – A study in which the researchers actively impose some treatment on the subjects and observe the response.

3. Think About It Think About It, p. 131.
Example – Charter School Achievement on the 2003 National Assessment of Educational Progress, page ii, 2nd paragraph.

4. Explanatory and Response Variables
Response variable – The variable that measures the outcome. It is believed to be affected by the explanatory variable.
Explanatory variable – The variable or factor that is thought to explain the observed outcome. It is believed to affect the response variable.

5. Let’s Do It!
Example 3.1, p. 132 – Explanatory Variable and Response Variable.
Let’s do it! 3.1, p. 134 – Explanatory Variable versus Response Variable.
Let’s do it! 3.2, p. 134 – Food Myths.
Example – Charter School Achievement on the 2003 National Assessment of Educational Progress, pages ii - iii.

6. 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.

7. Example
Example – Charter School Achievement on the 2003 National Assessment of Educational Progress, pages ii – iii.

8. Confound It!
Confounding variable – A variable whose effect cannot be separated from the effect of the explanatory variable.
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, the researchers did not properly control for the confounding variable.

9. Example
Example 3.3, p. 135 – Treating Multiple Sclerosis with Protein.
Let’s Do It! 3.3, p. 136 – Invasive versus noninvasive Cancer Treatments.
Example – Charter School Achievement on the 2003 National Assessment of Educational Progress, page 3, last paragraph.

10. Confounding Variables
Location
Not
Cen. City
Cen. City
lower
lower
Afr.-Amer.
Minority
Status
Non
Afr.-Amer.
lower
higher

11. Confounding Variables
Location
Not
Cen. City
Cen. City
lower
lower
Afr.-Amer.
lower
Minority
Status
Non
Afr.-Amer.
lower
higher
higher
lower
higher

12. Control by Minority Status
Non
Afr.-Amer.
Afr.-Amer.
lower
lower
Charter
lower
School
higher
higher
Public
higher

13. Control by Location Location Type of School Not Cen. City Cen. City
lower
lower
Charter
lower
Type of
School
higher
higher
Public
higher

14. Do Explanatory Variables 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!