fixed effect with ancova

Hi all,

I need calculate fixed effect with ancova analysis (spss or genstat).

My experiment was on fishes growth with differents diets (8): 15 fishes in each tank (each diet in triplicated, 3 tanks for each diet, total tanks:24). I need calculate p value for fixed effect: tanks and diets wit ancova analysis. whe i do it with just one effect (tank or diet separatedly) i found significants differences (this is the expected result), but when i do the analysis with both effect in the same ancova analysis (the correct method) i dont found significant diferences in diet...

I did read that the correct methos for this kind of problem coul be with blocks or nested method or just interaction tanks*diet

Any recomendation is valuable.


Super Moderator
If I understand correctly, then you have three true replicates (tanks) fish appear to be random effects and need to be treated as such in nested mised effects model where fish are nested in tank.