Hello everyone. Hope someone can help me.

I am running a Fisher exact 4 X 2 table analysis in SPSS. I have been informed its the correct test to use for the data. The problem is I have to conduct a power analysis to determine how many in both groups i need to be able to detect significance. How do i do that? I have downloaded gpower but I dont know how to calculate the different bits of information needed. Not sure if there is a different way to better way to know the sample size i need to have for the different levels of power effect sizes (small, medium, large). Hope someone can help as need to get this fixed as soon as i can if possible. Thanks in advance for the expertise in the group.