Re: fixed effect with ancova

Posted by Rich Ulrich on Aug 19, 2011; 8:44pm
URL: http://spssx-discussion.165.s1.nabble.com/fixed-effect-with-ancova-tp4713723p4716992.html

Maybe I'm missing something, but the results do make sense to me.

There is a large variation between Tanks; there is an interaction
between Tanks and Diet.  I'm not sure what you do have as error term
for D, but  T x D  looks reasonable.  That would undermine the difference
that was seen directly when looking at Diet alone. 
 
--
Rich Ulrich

> Date: Fri, 19 Aug 2011 12:42:04 -0700

> From: [hidden email]
> Subject: Re: fixed effect with ancova
> To: [hidden email]
>
> 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...
>