POL242 LAB MANUAL:  EXERCISE 3A

Crosstabulation with Nominal Variables

 

CONTENTS

 

PURPOSE

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MAIN POINTS

Crosstabulation

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Measures of Association: Nominal data--Phi and Cramer's V  

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INSTRUCTIONS: Crosstabulating Nominal Data

  1. Select one of the following Datasets for this exercise:  CCFRpop/elites, Macleans, CRIC, Euro2002.  

  2. Enter the Questionnaire for the chosen dataset from the Codebooks link on the POL242Y website.

  3. Hypothesize a relationship between two nominal variables in the dataset.
  4. Enter Webstats to select your chosen dataset.
  5. Perform separate trial-runs of the Frequency distribution for each of the variables. Based on the Frequency output, decide how to recode each variable and identify the missing values
  6. Set the Analysis in Webstats to Bivariate Crosstabs and hit Proceed.

  7. In 'Step 1,' enter the dependent variable first, followed by the independent variable.  Be sure to put the dependent or independent variable in the correct entry box.  If the variables are placed appropriately, the dependent variable will appear on the left of the crosstab and the independent variable will appear across the top (See diagram above).

  8. Select "Phi and Cramer's V (PHI)" in the 'Step 2' entry box.  This section lists others measures of association that you can choose tbut since we are working with nominal data select "Phi and Cramer's V (PHI)".
  9. Enter any recodes (if necessary) in 'Step 4' and hit Run.
  10. When evaluating the measures of association, you should look at only Phi for 2 by 2 tables and Cramer's V for other nominal tables.
  11. Determine whether there is a relationship between the variables based on the column-percentages in the crosstab.  Then, looking at the value of the measure of association, use the above guidelines to determine the strength of the relationship.
  12. Repeat the analysis until you find a set of variables with a relationship that has a moderate degree of association ( >.2).

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EXAMPLES

Example #1:  Using phi with two dichotomous variables

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Example #2: Cramer's V

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QUESTIONS FOR REFLECTION

  1. Did you discover a relevant relationship in your crosstab based on the column-percentages?  If so, was it evident in only one row of the table or in all rows? 
  2. Can you compare the magnitude of a Phi-value from one relationship to the magnitude of a Cramer's V value for another relationship?

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DISCUSSION

  1. When you find a cell that has a substantially different column-percentage from the other cells in that row, there are usually other rows in the table that also have a difference.  For example, if you find a difference in the column-percentage for cells A-B-C, then there is probably also a difference between D-E-F, or G-H-I.  This happens because the column-percentage in any given cell influences the column-percentage of the other cells in that column. 
  2. We can compare two values of the same measures readily.  But be cautious about comparing different measures of association to each other.  Eg., you should compare two measures of Phi to one other, but be cautious about comparing a Phi-value to a Cramer’s V value.

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