Chi-squared test for the relationship between two categorical variables - overview
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Chi-squared test for the relationship between two categorical variables | One sample $t$ test for the mean |
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Independent /column variable | Independent variable | |
One categorical with $I$ independent groups ($I \geqslant 2$) | None | |
Dependent /row variable | Dependent variable | |
One categorical with $J$ independent groups ($J \geqslant 2$) | One quantitative of interval or ratio level | |
Null hypothesis | Null hypothesis | |
H0: there is no association between the row and column variable More precisely, if there are $I$ independent random samples of size $n_i$ from each of $I$ populations, defined by the independent variable:
| H0: $\mu = \mu_0$
Here $\mu$ is the population mean, and $\mu_0$ is the population mean according to the null hypothesis. | |
Alternative hypothesis | Alternative hypothesis | |
H1: there is an association between the row and column variable More precisely, if there are $I$ independent random samples of size $n_i$ from each of $I$ populations, defined by the independent variable:
| H1 two sided: $\mu \neq \mu_0$ H1 right sided: $\mu > \mu_0$ H1 left sided: $\mu < \mu_0$ | |
Assumptions | Assumptions | |
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Test statistic | Test statistic | |
$X^2 = \sum{\frac{(\mbox{observed cell count} - \mbox{expected cell count})^2}{\mbox{expected cell count}}}$
Here for each cell, the expected cell count = $\dfrac{\mbox{row total} \times \mbox{column total}}{\mbox{total sample size}}$, the observed cell count is the observed sample count in that same cell, and the sum is over all $I \times J$ cells. | $t = \dfrac{\bar{y} - \mu_0}{s / \sqrt{N}}$
Here $\bar{y}$ is the sample mean, $\mu_0$ is the population mean according to the null hypothesis, $s$ is the sample standard deviation, and $N$ is the sample size. The denominator $s / \sqrt{N}$ is the standard error of the sampling distribution of $\bar{y}$. The $t$ value indicates how many standard errors $\bar{y}$ is removed from $\mu_0$. | |
Sampling distribution of $X^2$ if H0 were true | Sampling distribution of $t$ if H0 were true | |
Approximately the chi-squared distribution with $(I - 1) \times (J - 1)$ degrees of freedom | $t$ distribution with $N - 1$ degrees of freedom | |
Significant? | Significant? | |
| Two sided:
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n.a. | $C\%$ confidence interval for $\mu$ | |
- | $\bar{y} \pm t^* \times \dfrac{s}{\sqrt{N}}$
where the critical value $t^*$ is the value under the $t_{N-1}$ distribution with the area $C / 100$ between $-t^*$ and $t^*$ (e.g. $t^*$ = 2.086 for a 95% confidence interval when df = 20). The confidence interval for $\mu$ can also be used as significance test. | |
n.a. | Effect size | |
- | Cohen's $d$: Standardized difference between the sample mean and $\mu_0$: $$d = \frac{\bar{y} - \mu_0}{s}$$ Cohen's $d$ indicates how many standard deviations $s$ the sample mean $\bar{y}$ is removed from $\mu_0.$ | |
n.a. | Visual representation | |
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Example context | Example context | |
Is there an association between economic class and gender? Is the distribution of economic class different between men and women? | Is the average mental health score of office workers different from $\mu_0 = 50$? | |
SPSS | SPSS | |
Analyze > Descriptive Statistics > Crosstabs...
| Analyze > Compare Means > One-Sample T Test...
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Jamovi | Jamovi | |
Frequencies > Independent Samples - $\chi^2$ test of association
| T-Tests > One Sample T-Test
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Practice questions | Practice questions | |