Model Diagnostics Political Analysis II 1 Why should

Model Diagnostics Political Analysis II 1 Why should

Model Diagnostics Political Analysis II 1 Why should we care about unusual observations? Because they can drive our results and lead to misleading findings (especially in small samples) To improve our theory and statistical model Three types of unusual observations:

Regression outliers High leverage observations Influential observations 2 A useful tool: Residuals 3 Regression outliers

Regression outliers have extreme values for Y given their values on X For example, oil-rich non-democracies Why? Coding error, peculiarity Effect: limited, but they can increase our standard errors Detect: large studentized residuals (> | 2|) Fix: Check coding, revise theory

Fox (2008) 4 Example of regression outliers (1) Remember that Lijphart excluded India and Israel from his analysis because they had extreme values on the dependent variable of political stability and absence of violence? (i.e. univariate outliers) 5

Example of regression outliers (2) Remember that Lijphart excluded India and Israel from his analysis because they had extreme values on the dependent variable of political stability and absence of violence? (i.e. univariate outliers) Only Israel, however, is a regression outlier! 6 High leverage observations

High leverage observations have Detect: hat values (measure extreme values on one or more based on the fitted/Y-hat values) independent variables. Effect: They can change the estimate of regression coefficients (if they dont follow the pattern of the data) Fox (2008)

7 Example of high leverage observations Lijphart described India as an extreme outlier, but it is actually a high leverage observation. 8 Example of high leverage

observations Lijphart described India as an extreme outlier, but it is actually a high leverage observation. 9 Example of high leverage observations We can see this clearly when we look at Indias very high hat-values.

10 Influential observations Influential observations have extreme Detect: studentized residuals versus values for X and Y leverage, Cooks Distance Influence = Outlierness X Leverage Fix: check coding, dummying out, re-run the model without the Effect: removing them from the model

observation(s) and compare results significantly changes the direction, strength, or significance of the results Fox (2008) 11 Example of influential observations No influential observations in Lijpharts sample

India: high hat-values, but small residuals Israel: large residuals, but low hatvalues We find influential observations in the lower-right corner and upperright corner (not shown here). 12

The infamous butterfly ballot Wand et al. (2001) show that more than 2,000 Democrats voted for Buchanan in Palm Beach County, a typically Democratic county, due to the butterfly ballot. This type of ballot was only used in this county and only for election-day for president. As a result, George W. Bush, and

not Al Gore, won Florida and the presidency. Kellstedt and Whitten (2013) 13 14 What should we do with unusual observations?

Dont ignore it Learn to understand why an observation is unusual If it is a coding error, recode it; if thats not possible, drop it Learn more about the observation and improve your model Transform the data (e.g. log transformation) More on nonlinear relationships next week 15 Multicollinearity Multicollinearity is the high

correlation between two or more independent variables. For example, height and weight Ideology and party ID Effect: larger standard errors ( Type II errors), unstable coefficients Detect: variance-inflation factor VIF (VIF > 10 are problematic) Fix: not easy, but increasing sample

size, centering variables, or combining variables into an index can help. Kellstedt and Whitten (2013: 238) 16 More articles on influential observations Fails and Krieckhaus (2010). Colonialism, Property Rights and the Modern World Income Distribution. British Journal of Political Science, 40(3), 487503. Data:

https://sites.google.com/a/oakland.edu/mfails/research/colonialism-prope rty-rights-and-the-modern-world-income-distribution Wand et al. (2001). The Butterfly Did It: The Aberrant Vote for Buchanan in Palm Beach County, Florida. American Political Science Review, 95(4), 793810. Data: h ttps://dataverse.harvard.edu/dataset.xhtml?persistentId=hdl:1902.1/1038 9 17

Recently Viewed Presentations

  • Ozymandias: A Study - Maite Space

    Ozymandias: A Study - Maite Space

    Ozymandias: A Study Written by Percy Bysshe Shelley Essay by Sanger Rainsford Setting in Ozymandias The setting of Ozymandias is a desolate wasteland Deserts represent the sands of time and the expanse of time before and after Ozymandias.
  • DE-PBS Cadre Meeting - University of Delaware

    DE-PBS Cadre Meeting - University of Delaware

    DelTech(Terry Campus) - Room 400A Why Team members and project coaches can share materials, ideas, obstacles and strengths in implementation efforts to help strength their current DEPBS programming.
  • Whats Causing Increased Rx Costs ? (and more

    Whats Causing Increased Rx Costs ? (and more

    "Airtight" (it means what you intend) Auditable (it identifies an agreed-upon methodology) Enforceable (it specifies how damages are to be calculated - and paid - if the PBM breaches the guarantee) You Must Demand A Contract with Numerous Guarantees, &...
  • Lake Charles Harbor and Terminal District

    Lake Charles Harbor and Terminal District

    PORT OF LAKE CHARLES CITY DOCKS 11 USAID/USDA approved transit sheds 13 Ship Berths 1.3 million square feet waterfront storage 600,000 square feet of warehousing behind the waterfront Vegetable oil packaging plant Automated Bag Handling Facility Public Grain Elevator PUBLIC...
  • Rollback by Reverse Computation - Cornell University

    Rollback by Reverse Computation - Cornell University

    Times New Roman Courier New Default Design Rollback by Reverse Computation Research on Reversible Computation Georgia Tech Time Warp (GTW) Optimistic Synchronization State-Saving Advantages of Reversible Computing Reversing C Programs RCC: Reverse C Compiler Saving Destroyed State Data: The Tape...
  • State and Local Politics: Institutions and Reform

    State and Local Politics: Institutions and Reform

    Minority Representation Political incorporation of non-whites slow in cities & counties - Why important? Virtues of "descriptive" representation
  • CHAPITRE I: concept et domaine

    CHAPITRE I: concept et domaine

    Approche stratégique; Autre modèle d'analyse : la Matrice BCG (Boston Consulting Group), son action va vers Les organisations confrontées à la question de leur développement face à la concurrence et aux différentes évolutions de leurs environnements et qui devraient entrevoir...
  • OXYGEN THERAPY in a Bottle - Oxyrich

    OXYGEN THERAPY in a Bottle - Oxyrich

    Oxyrich is produced in Australia without the use of chemical salts or other compounds. The manufacturing process is Bio Electrical Manufacture (BEM). Oxyrich is an oxygen supplement The sole active ingredient in Oxyrich is dissolved diatomic oxygen molecules. Are there...