1. Comparing Two Variables
PS 225 Lecture 16
Comparing Two Variables

2. Studying Combinations of Variables
Independent variable – variable believed to cause changes in the dependent variable
Dependent variable- variable believed to reflect changes caused by the independent variable

3. Example
Example: Colder temperatures may cause changes in the amount of ice formed.
Temperature is the independent variable
Amount of ice is the dependent variable

4. Comparing Data
More than one piece of information about each subject in the study
Plotted in a table with columns representing one variable and rows representing the other variable
Contents of the cells may be the frequency, percentage or mean of values you are interested in

5. Cross Tabulation

6. Chi-Squared Test Test for independence Can be used for nominal data
Nonparametric test

7. Crosstabulation
Independent Variable
Dependent Variable

8. Crosstabulation
Counts

9. Crosstabulation
Percentages

10. Crosstabulation
Marginal Totals

11. Crosstabulation Formulas
Marginal frequencies
Sum of all frequencies in column
Sum of all frequencies in row
Percentages
(Frequency in Cell)/(Frequency for Column) *100
Marginal Percentages
Row = (Frequency in Row)/(Total Frequency)*100
Column = Sum of Percentages in Column

12. Chi-Squared Statistic

13. SPSS Assignment
Consider the actual GSS data on Gender and Political Party
Create a crosstabulation comparing the two variables with gender as the independent variable and political party as the dependent variable
Answer the question: Does there appear to be any relationship between gender and political party affiliation? What is the relationship between gender and political party affiliation?