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Review.

Review. POL 242 – 2008-09. Strong Correlation. Positive or Negative?. Correlations - Views. Adding a 3 rd (& 4 th & 5 th ) Variable. More than one independent variable. How can we characterize the relationship between these independent variables and the dependent variable. Additive

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Review.

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  1. Review. POL 242 – 2008-09

  2. Strong Correlation. Positive or Negative?

  3. Correlations - Views

  4. Adding a 3rd (& 4th & 5th) Variable • More than one independent variable. How can we characterize the relationship between these independent variables and the dependent variable. • Additive • Antecedent • Intervening

  5. Original Relationship Can Change • Did the relationship between X and Y change at different levels of the new variable? • Did the relationship get weaker? stronger? • Did the sign change or stay the same? • Focus on the relationship between X & Y, not on how the new variable affects the independent variable.

  6. Control Tables • Look at a bivariate relationship at different levels of the [new] control variable. • Observe appropriate measures of association • How can you gauge if there is elaboration?

  7. What to do if there is elaboration • If the relationship between x and y dramatically changes at different values of z, then you want to create an interaction term. • Need a new line with a different slope for those values of z. • Remember to include the original variables in the equation as independent variables.

  8. Interpreting Interactions • Two Dummies • EX: Immigrant and univ graduate. • Three coefficients • Immigrant • Univ Graduate • Immigrant * Univ Graduate (Interaction) • Immigrant= immigrants w/o univ. diploma • Univ Grad = non-immigrant graduates • Interaction = immigrants WITH diploma

  9. Interpreting Interactions - II • Dummy + Ordinal, Ex: Race (white = 1) and Political Interest (ordinal 5 pt scale) • Three coefficients • White = effect of being white with no political interest on DV • Political Interest = effect of political interest on DV for non-whites only • Interaction = Effect of political interest on DV for whites only.

  10. Regression – To Do • Need to run a regression • Interpret that regression • B! • Significance • Could there be a multicollinearity problem? • Tolerance / VIF • Beta • R-squared

  11. Advanced Regression - To Do • Recode variables • Create a dummy variable • Rules of creating dummies • Interpretation of dummies • Create an interaction term • Interpretation of all terms in equation • Run a regression on sub-sets of sample

  12. When should you not run a Regression • OLS = Ordinary Least Squares Regression • Best for interval dependent variables or ordinal variables with at least five different categories. • Also imposes other restrictions about distribution of variable and the error term • You can run a logit regression on a dichotomous dependent variable.

  13. Logit Rules • You can run a logit regression on a dichotomous dependent variable. • But you cannot directly interpret the coefficients. • You can tell whether the effect is negative or positive, and whether the effect is statistically significant. • Remember how to see if the model did a good job of predicting the actual results.

  14. Final Product • Evaluate your hypothesis or hypotheses. • If evidence is consistent with your hypothesis, reject your null hypothesis. • Make your point(s) very clearly. • Do your readers know everything they need to know? • And remember that they may not be interested in EVERYTHING.

  15. Details • Everyone come to class next week (12:15 papers to the office will be marked LATE). • If you are not taking test in-class, you should be in location with internet access. • Everyone should fill out an evaluation form. • TAs • Professors • Papers go to Turnitin.com

  16. Details – Open Book Section • Start on internet with Webstats. Can reference class materials (labs, lectures), but your time is limited. • May not converse with others (talk, IM, etc) or consult other people’s analyses. • Can bring disk or USB memory key to save Word document for your answers (can also save to My Documents) • At end, email answers YOURSELF and to pol242y@canada.com and upload to Blackboard.

  17. Second Half – Closed Book • 28 questions (1/2 value of test) is closed book. NO INTERNET. • Multiple choice, true/false, fill in the blank and some short answers. • Recommended: 45 minutes • Difficulty: should be similar to labs and worksheets. • If you are well-prepared, I expect you will do well.

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