13.2 Chi-Square Test for Independence

The chi-square test for independence is a powerful tool for analyzing relationships between categorical variables. It helps determine if there's a significant association between two variables by comparing observed frequencies to expected frequencies if the variables were independent.

This test is crucial for understanding patterns in data, especially in business contexts. By constructing contingency tables, calculating the chi-square statistic, and interpreting results, we can uncover valuable insights about customer preferences, market trends, and other important categorical relationships.

Chi-Square Test for Independence

Appropriateness of chi-square test

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• Used when analyzing relationship between two categorical variables (nominal or ordinal)
  • Nominal has no inherent order (gender, color, product category)
  • Ordinal has natural order but no fixed interval (education level, satisfaction rating, income bracket)
• Assesses significant association between variables
  • Null hypothesis ($H_0$): Variables are independent, no association
  • Alternative hypothesis ($H_1$): Variables are dependent, association exists
• Requires data from single population with each subject classified on both variables simultaneously
  • Cannot combine data from separate populations or different time periods

Construction of contingency tables

• Contingency table is matrix displaying frequency distribution of variables
  • Rows represent categories of one variable (age groups)
  • Columns represent categories of other variable (preferred product)
  • Each cell contains observed frequency (count) for combination of categories
• Calculate expected frequency for each cell assuming null hypothesis is true
  • Formula: $E_{ij} = \frac{(Row_i \text{ Total}) \times (Column_j \text{ Total})}{Overall \text{ Total}}$
    • $E_{ij}$: Expected frequency for cell in row $i$ and column $j$
    • $Row_i \text{ Total}$: Total frequency for row $i$ (sum of all cells in row)
    • $Column_j \text{ Total}$: Total frequency for column $j$ (sum of all cells in column)
    • $Overall \text{ Total}$: Total sample size (sum of all cell frequencies)
  • Compares observed frequencies to expected frequencies if variables were independent

Calculation of chi-square statistic

• Chi-square test statistic ($\chi^2$) measures difference between observed and expected frequencies
  • Formula: $\chi^2 = \sum_{i=1}^{r} \sum_{j=1}^{c} \frac{(O_{ij} - E_{ij})^2}{E_{ij}}$
    • $O_{ij}$: Observed frequency for cell in row $i$ and column $j$
    • $E_{ij}$: Expected frequency for cell in row $i$ and column $j$
    • $r$: Number of rows in contingency table
    • $c$: Number of columns in contingency table
  • Larger differences between observed and expected frequencies lead to higher $\chi^2$ values
• Degrees of freedom (df) for chi-square test for independence
  • Formula: $df = (r - 1)(c - 1)$
  • Represents number of cells that can vary freely while maintaining row and column totals
  • Used to determine critical value and p-value from chi-square distribution

Interpretation of chi-square results

• Compare calculated chi-square test statistic to critical value from chi-square distribution
  • Use degrees of freedom and desired significance level (usually $\alpha = 0.05$)
  • If test statistic exceeds critical value, reject null hypothesis
• p-value: Probability of observing test statistic as extreme as calculated value, assuming null hypothesis is true
  • If p-value is less than chosen significance level, reject null hypothesis
• Rejecting null hypothesis implies significant association between variables
  • Variables are dependent, not independent
  • Observed frequencies differ significantly from expected frequencies under assumption of independence
• Failing to reject null hypothesis suggests no significant association between variables
  • Variables are independent
  • Observed frequencies are close to expected frequencies under assumption of independence
• Effect size measures strength of association (Cramer's V or phi coefficient)
  • Values range from 0 (no association) to 1 (perfect association)
  • Interpretation depends on size of contingency table (number of rows and columns)

Assumptions and Considerations

• Independence: Observations within each sample must be independent
  • Randomly selected from population
  • No relationship between observations in different cells (one observation cannot influence another)
• Sample size: Expected frequencies in each cell should be sufficiently large
  • At least 80% of cells should have expected frequencies of 5 or more
  • If assumption is violated, consider using Fisher's exact test instead
• Avoid excessive number of categories in variables
  • May lead to small expected frequencies and violate sample size assumption
  • Combine categories if necessary to meet assumptions
• Report results clearly and accurately
  • Include contingency table, chi-square test statistic, degrees of freedom, p-value, and effect size
  • Interpret results in context of research question and hypotheses
  • Discuss limitations and potential confounding variables that may affect interpretation