Political Science 30: Political Inquiry

Isolating Causal Factors Using Tables and Graphs Political Science 30:Political Inquiry

Isolating Causal Factors • Using Tables to “Hold Constant” a Confounding Variable • Income and voting • Education as a confound • Using Graphs to “Hold Constant” A Confound or Another Causal Factor • California Connected charts

Using Tables to “Hold Constant” a Confounding Variable • We want to test the hypothesis that “Americans who have higher incomes are more likely to vote.” • IV: Level of Income • DV: Likelihood of Voting Income Likelihood of Voting

Using Tables to “Hold Constant” a Confounding Variable • Wolfinger and Rosenstone present evidence of the bivariate correlation between income and voting.

Using Tables to “Hold Constant” a Confounding Variable • But wait! Education is a possible confounding variable because (Lecture #2): • Education changes the likelihood of voting • Education is correlated with income • Education is causally prior to income Education Voting Income

Using Tables to “Hold Constant” a Confounding Variable • Thought Experiment: What if we could compare the voting behavior of two people who had the same level of education but different incomes? • Me vs. my friends who went to law school • This analysis “holds constant” education • It isolates the effect of income on voting • Same logic as experiments, natural experiments, and quasi-experiments

Using Tables to “Hold Constant” a Confounding Variable • Wolfinger and Rosenstone hold education constant by using a two-way table.

Using Tables to “Hold Constant” a Confounding Variable • Once they held education constant, they found that “rock bottom poverty seems to depress turnout somewhat. Beyond that, income does not have much effect on turnout. (p. 26)” • Holding constant (or “controlling for”) a confound or another causal factor often teaches us about the bivariate correlation.

Using Graphs • A scatterplot is one way to tell if two interval or ratio variables are correlated.

You can connect points with lines in a scatterplot

Using Graphs • Line graphs tell us about trends over time, but watch the scale!

Using Graphs

Line Graphs Can Show Difference in Proportions

Using Graphs Bar graphs are helpful when your independent variable is nominal

Using Graphs to “Hold Constant” A Confound or Another Causal Factor • I wanted to see whether the presence of term limits (IV #1) made California’s Legislature less likely to alter the Gov’s budget (DV). Term Limits % altered Divided/Unified Government (IV #2)

Using Graphs to “Hold Constant” A Confound or Another Causal Factor

Political Science 30: Political Inquiry