Re: fixed effect with ancova

Posted by ro on Aug 19, 2011; 7:42pm
URL: http://spssx-discussion.165.s1.nabble.com/fixed-effect-with-ancova-tp4713723p4716821.html

Hi Gene,

I have to adjust a lineal model for genetic variance, i have to adjust one model for this purpose, maybe you are thinking in the covariate (exist other variable in my experiment that i did measure: lenght, before this analysis i did run one correlation analysis and have a very good correlation with weight...)
then for my genetic lineal model i have to found fixed effect and covariate to input to the model, for this reason ancova. i have measured for all fishes (360 animals) each 3 months and the end of experiments...

i know that i get diet and tank main effects and diet-tank interaction. but i have problems when i run this model, when i run ancova whit only diet or tank effects separatedly i have p levels <0.001 for both fixed effects (expected results)

but when i run
weight= u+ diet_effect+ tank_effect+error
i get:
tank_effect: p.value: <0.01
diet_effect: p.value: 0.89
diet_effect*tank_effect: p.value: <0.01
covariate: p.value: <0.01

so why  diet_effect disapear in ancova analysis?, i will hope get both fixed effect...