This often is determined by chance. Chance would give all proportions or totals equal likelihood of happening. Chi-square distributions vary in shape slightly depending on how many frequencies or proportions it needed to consider. Because chi-square can be used to test more than 2 proportions, we can use the procedure to study the relationship between categorical variables. When used in this sense, the chi-square is known as a test of independence.
All margins sum to the total sample size.
As which z and t, the critical area under the tail is usually .05.