[R-sig-ME] FW: random effect

[Joerg Luedicke]

Joana Martelo asked:
"However, do you think that considering year a fixed effect will
affect the relationship between the other explanatory variables and
the response?"

Entering an additional variable in a regression model will always
change the effects of other predictor variables on the outcome that
were estimated in a previous model. In your case, entering year as a
binary predictor essentially means that you constrain your other
effects to be the same in both years. If you have reason to believe
that the effects are different across the two years, you can interact
year with your other predictor variables and thereby relax the
assumption that the effects are the same in both years.

Joerg

On Sun, Oct 7, 2012 at 11:29 AM, joana martelo <jmmartelo at fc.ul.pt> wrote:
> Dear list
>
>
>
> I’m modeling fish activity data with a gaussian distribution for scores
> obtained from Principal Component Analysis. My explanatory variables are
> group size, fish length, temperature and year. Because year has only two
> levels I know I can’t use it as a random effect. However, do you think that
> considering year a fixed effect will affect the relationship between the
> other explanatory variables and the response? If the answer is yes, do you
> know of other statistical technique I could use?
>
> Thank you
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> All the best
>
> Joana
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> Joana Martelo, PhD Student
>
> Centro de Biologia Ambiental
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> Departamento de Biologia Animal
>
> Faculdade de Ciências, Edificio C2,5ºPiso,Sala 2.5.15B
>
> 1749-016 Lisboa, Portugal
>
> http://ffishgul.fc.ul.pt
>
>
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