Chisquared test for the relationship between two categorical variables: overview
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Chisquared test for the relationship between two categorical variables 


Independent /column variable  
One categorical with $I$ independent groups ($I \geqslant 2$)  
Dependent /row variable  
One categorical with $J$ independent groups ($J \geqslant 2$)  
Null hypothesis  
 
Alternative hypothesis  
 
Assumptions  
 
Test statistic  
$X^2 = \sum{\frac{(\mbox{observed cell count}  \mbox{expected cell count})^2}{\mbox{expected cell count}}}$
where 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  
Sampling distribution of $X^2$ if H0 were true  
Approximately a chisquared distribution with $(I  1) \times (J  1)$ degrees of freedom  
Significant?  
 
Example context  
Is there an association between economic class and gender? Is the distribution of economic class different between men and women?  
Pratice questions  