fixed effect with ancova

Posted by ro on Aug 18, 2011; 10:57pm
URL: http://spssx-discussion.165.s1.nabble.com/fixed-effect-with-ancova-tp4713723.html

Hi all,

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

My experiment was on fishes weight 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. the correct or desired model is

weight= Intercept + Diet_Effect + tank_effect+ epsilon

when i do this analysis 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 methods for this kind of problem coul be with blocks or nested method or just interaction tanks*diet, i hope capture variance produced by tanks

my spredsheet looks like

diet     tank     weight
1        1
1        2
1        3
2       4
2       5
2       6
3       7
3       8
3       9
...
Any recomendation is valuable.