Re: fixed effect with ancova

Posted by Art Kendall on Aug 20, 2011; 12:28pm
URL: http://spssx-discussion.165.s1.nabble.com/fixed-effect-with-ancova-tp4713723p4718349.html

You have a finding that the relation between diet and weight differs by tank.
IF the finding does not make sense then
do a graph with weight on the vertical, tanks on the horizontal with dots for the diet by tank means.
connect the three dots for each diet.  Your results are saying that those "curves" are not parallel.
Does it look that way visually?

It may be that one cell has a data error, maybe a slipped decimal point.  Did you proofread your data?

However, from your present description, it sound like you missed putting a repeated measure factor in your model (time with 4 or 5 levels).
Between group factors: diet (8) and tank (3). (24 cells with 15 fish in each). Within fish specimen repeated weights (4?, 5?).  You also have some continuous variables that describe the 360 fish.  Are they constant over the repeats or do they vary over the repeats? I.e., do you have 360 or 1440 measures of the covariates?


Art Kendall
Social Research Consultants

On 8/19/2011 3:42 PM, ro wrote:

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...

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