I have two questions in regards to this data and possible confounds:

1) Within each condition participants differed on a variable say X (not the condition they were in) that had 3 possible values. What would be the best way to control for X to eliminate it as a confound? If i do a chi-square test of independence then i can only find evidence whether proportions for each condition are significantly different or not (not evidence that the null hypothesis is true that the proportion of people for each value of x is roughly equal for each condition) right? Someone suggested log-linear analysis and someone also suggested stratified analysis (but I think my strata will have too small of a sample size then...)

2) After the experiment people had the option of filling in a post-survey. I would like to determine that the people who chose to fill in the survey were evenly distributed among the 3 conditions (can i use a chi-squared test of homogeniety for this and if the result is not significant state there were no significant differences in the number of people per condition who filled out the post-survey and thus the post-survey results are not biased to be representative of a particular condition?

Thanks for any help!