Which statistical test is better? (chi square) I am wondering about the chi square test and what will give me more reliable/better results.
I have four categories
Aa
Ab
Ba
Bb
I want to test if these four categories are in a 1:1:1:1 ratio, however; I also can add up the Ba+Bb and Aa + Ab categories and test if these follow a 1:1 ratio. Which test is more accurate?

If you want to see if they occur in a 1:1:1:1 ratio, you would have to test the 4 frequencies simultaneously.
For example if

*

*Aa = 50

*Ab = 0

*Ba = 25

*Bb = 25

then clearly the four categories are not occurring at a 1:1:1:1 ratio.  But the second test would show that Aa+Ab and Ba+Bb occur at a 1:1 ratio.
The choice in tests depends on which type of difference you want to detect.
