# Non-homogeneity in 2-way ANOVA in SPSS

I am trying to determine differences in fish weights from 1st to 2nd sampling between a control group and 4 different treatment groups. My groups are unbalanced (control = 1 group, t1= 2 groups, with t2,t3,t4 = 3 groups). Originally I was told this shouldn't be a problem.

I've run a 2-way ANOVA and the Levene's test in SPSS is significant (0.017) but ANOVA is (0.018) and the significant factor is (.001). Is there a way to adjust for this or do I need to run a different stats test?

Otherwise I was thinking I may need to simplify and combine all the treatment groups and run a t-test, then run another t-test for the control to demonstrate there are differences between treated and non-treated fish. What do you think?

Ok so there are a total of 21 tanks of fish i.e. n= 21. We didnt have room for 22 tanks so one treatment group only had to 2 tanks. We attempted to run everything in triplicate so 3 of the treatment groups are triplicated. 1 treatment group is duplicated, and 1 group is the control (single). The weights of fish are tanks averages. The largest difference is a total of 22 grams, the smallest is a difference of less than 1 between tanks

• This isn't at all clear. Can you say more about your study & your data? How can treatment group 1 have 2 groups? How much heterogeneity do you have? Ie, what are the variances of the groups w/ the largest & smallest variances? – gung - Reinstate Monica Jan 23 '14 at 2:31
• Can you give a sense of what the total overall $n$ is in this sample? – AdamO Jan 23 '14 at 5:33
• Ok so there are a total of 21 tanks of fish i.e. n= 21. We didnt have room for 22 tanks so one treatment group only had to 2 tanks. We attempted to run everything in triplicate so 3 of the treatment groups are triplicated. 1 treatment group is duplicated, and 1 group is the control (single). The weights of fish are tanks averages. The largest difference is a total of 22 grams, the smallest is a difference of less than 1 between tanks. – user37695 Jan 27 '14 at 23:38
• @user37695 how many fish in each tank? Can they be measured at baseline and then at follow-up? Or can they be measured multiple times (as opposed to, say, being sacrificed to measure liver volume) – AdamO Jan 28 '14 at 19:13
• Hi Adam, they are sacrificed at time point 1 (start) and 2 (end). I have already run some a GLMM and determined that for mortality rates there are insignificant cluster effects. I spoke to the Uni stats guys here and he said the averaging of the weight should not be a problem. Each tank had 6 fish measured at the start and the end of the trial. – user37695 Jan 30 '14 at 4:27