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4. The Chi-Square Test for Goodness of Fit - No Difference from a Known Population

Suppose you are reading a study conducted in the year 2000 about welfare recipients in the United States. The authors report the following frequency data on the household size of the 1,844 welfare recipients in their random sample:

\begin{tabular}{ccccc}
\hline \multicolumn{5}{c}{ Observed Frequencies } \\
Household Size & 5-or-more-person & 4-person & 3-person & 2-person & 1-person \\
\hline 203 & 277 & 301 & 550 & 513 \\
\hline
\end{tabular}

You wonder if welfare recipients tend to live in different-sized households than the US population at large. You obtain the following data from the 2000 census:

\begin{tabular}{|c|c|c|c|c|}
\hline \multicolumn{5}{|c|}{ Percent Distribution of US Households by Size } \\
Household size & 5-or-more-person & 4-person & 3-person & 2-person & 1-person \\
\hline [tex]$10.83\%$[/tex] & [tex]$14.20\%$[/tex] & [tex]$16.53\%$[/tex] & [tex]$32.63\%$[/tex] & [tex]$25.81\%$[/tex] \\
\hline
\end{tabular}

[Source: Mobbe F. A. Scoops, N. (2002). Census 2000 Special Reports: Demographic Trends in the 20th Century. US Census Bureau.]

You use a chi-square test for goodness of fit to see how well the sample of welfare recipients fits the census data. What is the most appropriate null hypothesis?

A. The distribution of household sizes among welfare recipients is equal across the five household-size categories.
B. The distribution of household sizes among welfare recipients is the same as that provided by the census data.
C. The distribution of household sizes among welfare recipients is different from that provided by the census data.
D. The distribution of household sizes among welfare recipients is not equal across the five household-size categories.


Sagot :

To determine if the distribution of household sizes among welfare recipients is the same as the distribution provided by the 2000 census data, we can use a chi-square test for goodness of fit.

1. State the hypotheses:
- Null Hypothesis (H₀): The distribution of household sizes among welfare recipients is the same as that provided by the census data.
- Alternative Hypothesis (Hᴀ): The distribution of household sizes among welfare recipients is different from that provided by the census data.

2. Observed frequencies from the study sample:
- 1-person households: 513
- 2-person households: 550
- 3-person households: 301
- 4-person households: 277
- 5-or-more-person households: 203

3. Expected frequencies based on census data (percentages):
- Total number of observations: 1844
- Expected percentages:
- 1-person households: [tex]\( 25.81\% \)[/tex]
- 2-person households: [tex]\( 32.63\% \)[/tex]
- 3-person households: [tex]\( 16.53\% \)[/tex]
- 4-person households: [tex]\( 14.20\% \)[/tex]
- 5-or-more-person households: [tex]\( 10.83\% \)[/tex]

4. Calculate expected frequencies:
- 1-person households: [tex]\( 1844 \times \frac{25.81}{100} = 475.61 \)[/tex]
- 2-person households: [tex]\( 1844 \times \frac{32.63}{100} = 601.57 \)[/tex]
- 3-person households: [tex]\( 1844 \times \frac{16.53}{100} = 304.77 \)[/tex]
- 4-person households: [tex]\( 1844 \times \frac{14.20}{100} = 261.85 \)[/tex]
- 5-or-more-person households: [tex]\( 1844 \times \frac{10.83}{100} = 199.20 \)[/tex]

5. Perform the chi-square test for goodness of fit:
- The chi-square statistic (χ²) is calculated as follows:
[tex]\[ \chi^2 = \sum \frac{(O_i - E_i)^2}{E_i} \][/tex]
where [tex]\( O_i \)[/tex] is the observed frequency and [tex]\( E_i \)[/tex] is the expected frequency for each category.

6. Calculate the chi-square statistic and p-value:
- From the given solution:
[tex]\[ \chi^2 = 8.31 \quad (\text{rounded to two decimal places}) \][/tex]
[tex]\[ p = 0.081 \quad (\text{rounded to three decimal places}) \][/tex]

7. Decision:
- Typically, we use a significance level of 0.05. If the p-value is less than the significance level, we reject the null hypothesis.
- In this case, [tex]\( p = 0.081 \)[/tex], which is greater than 0.05.

8. Conclusion:
- Since the p-value is greater than the significance level, we do not reject the null hypothesis. There is not enough evidence to conclude that the distribution of household sizes among welfare recipients is different from the distribution provided by the census data.

Therefore, the most appropriate null hypothesis is:
- "The distribution of household sizes among welfare recipients is the same as that provided by the census data."