Analysis of Nominal-Scale Data

Analysis of Nominal-Scale Data

 

### Unit II: Analysis of Nominal-Scale Data

#### A. **Rationale**

Nominal-scale data refers to data that is categorized without any quantitative value or inherent ranking between the categories. These variables represent distinct groups or types, such as gender, ethnicity, religion, or political affiliation. The key rationale for analyzing nominal data is to summarize and compare proportions or frequencies within different categories, as well as to assess relationships between these categories. Since nominal data does not involve a hierarchy or order, only frequency-based analyses are suitable for such data.

Nominal data is often visualized using bar charts or pie charts to show proportions, and it is analyzed using techniques such as frequency tables and contingency tables to explore relationships between variables.

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#### B. **Univariate Data Analysis: One-Way Frequency Table**

A **one-way frequency table** is used in univariate analysis (the analysis of a single variable) to display the number of occurrences for each category within a nominal variable. This helps in summarizing how often each category appears in a dataset.

For example, if you are analyzing a dataset on political affiliation with categories such as Democrat, Republican, and Independent, a one-way frequency table would display the count of respondents in each category:

| Political Affiliation | Frequency |

|-----------------------|-----------|

| Democrat              | 100       |

| Republican            | 120       |

| Independent           | 80        |

This table provides a clear, simple representation of how the data is distributed across categories.

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#### C. **Bivariate Data Analysis: Two-Way Frequency Table and Chi-Square Test**

**Two-Way Frequency Table (Contingency Table)**:

A two-way frequency table, also known as a **contingency table**, is used to explore the relationship between two nominal variables. It shows how frequently each combination of categories occurs. For example, a contingency table might compare **political affiliation** with **gender**:

|                | Democrat | Republican | Independent | Total |

|----------------|----------|------------|-------------|-------|

| Male           | 50       | 70         | 30          | 150   |

| Female         | 50       | 50         | 50          | 150   |

| Total          | 100      | 120        | 80          | 300   |

This table can help sociologists assess whether there is an association between gender and political affiliation.

**Chi-Square Test**:

The chi-square test is a statistical test used to determine whether there is a significant association between two nominal variables. It compares the observed frequencies in the contingency table to the expected frequencies (what would occur if there were no association between the variables).

The formula for the chi-square statistic (χ²) is:

\[

\chi^2 = \sum \frac{(O - E)^2}{E}

\]

Where:

- **O** = Observed frequency

- **E** = Expected frequency (calculated under the assumption of no relationship between the variables)

If the calculated chi-square value exceeds a certain threshold (based on the degrees of freedom and significance level), the null hypothesis (no relationship between the variables) is rejected, indicating that a significant association exists.

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#### D. **Level of Significance (Measures of Strength of Relationship)**

In hypothesis testing, the **level of significance** (denoted by **α**) is the threshold for determining whether to reject the null hypothesis. Typically, α is set at 0.05, meaning that there is a 5% risk of rejecting the null hypothesis when it is actually true (a Type I error).

- **P-value**: The p-value indicates the probability of observing the test results under the assumption that the null hypothesis is true. If the p-value is less than the level of significance (e.g., p < 0.05), the null hypothesis is rejected.

- **Cramér's V**: This is a measure of the strength of association between two nominal variables. Cramér's V ranges from 0 (no association) to 1 (perfect association). It is derived from the chi-square statistic and accounts for the size of the table.

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#### E. **Interpretation**

The interpretation of results from chi-square tests or frequency tables involves determining whether there is a statistically significant relationship between variables. If the chi-square test shows significance (p < 0.05), it indicates that the observed relationship between the variables is unlikely to have occurred by chance.

- In the context of a two-way table, the interpretation involves looking at whether the distribution across categories deviates from what would be expected under the assumption of no association.

- In addition, the strength of the relationship (using Cramér's V) can help in determining whether the relationship, even if significant, is weak or strong.

For example, in the political affiliation and gender analysis, if the chi-square test is significant, it may suggest that gender is related to political affiliation in the sample.

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#### F. **Inference**

Inference in nominal-scale data analysis refers to making generalizations about a population based on the analysis of a sample. After conducting tests like chi-square, sociologists can infer whether the relationships observed in the sample likely hold true for the larger population. This is done while acknowledging the limitations of the data, including sample size, potential biases, and random error.

For example, if the chi-square test reveals a significant relationship between gender and political affiliation in the sample, a researcher might infer that gender plays a role in political affiliation in the broader population, assuming the sample is representative.

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### **Readings** for this Unit:

1. **Blalock, H.M.** (1969). *Nominal Scales: Proportions, Percentages, and Ratios* (Chapter 3, pp. 31-40): This reading focuses on the application of proportions, percentages, and ratios in the analysis of nominal data, providing a detailed understanding of how these tools can summarize nominal-scale data effectively.

2. **Blalock, H.M.** (1969). *Nominal Scales: Contingency Problems* (Chapter 15, pp. 275-316): This chapter delves into the challenges of analyzing relationships between nominal variables using contingency tables and offers solutions for accurately interpreting contingency problems in sociological research.

These readings will deepen your understanding of nominal-scale data analysis and its application in sociological research. Let me know if you'd like further elaboration on any of these topics!