[R-sig-ME] Model diagnostics show slope in residuals plot and slope on the observed vs fitted plot is different than y = x

[Thierry Onkelinx]

Dear Carlos,

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Best regards,

ir. Thierry Onkelinx
Instituut voor natuur- en bosonderzoek / Research Institute for Nature and
Forest
team Biometrie & Kwaliteitszorg / team Biometrics & Quality Assurance
Kliniekstraat 25
1070 Anderlecht
Belgium

To call in the statistician after the experiment is done may be no more
than asking him to perform a post-mortem examination: he may be able to say
what the experiment died of. ~ Sir Ronald Aylmer Fisher
The plural of anecdote is not data. ~ Roger Brinner
The combination of some data and an aching desire for an answer does not
ensure that a reasonable answer can be extracted from a given body of data.
~ John Tukey

2016-10-02 17:57 GMT+02:00 Carlos Familia <carlosfamilia op gmail.com>:

> Hello,
>
> I have in hands a quite large and unbalanced dataset, for which a Y
> continuous dependent variable was measured in 3 different conditions (C)
> for about 3000 subjects (ID) (although, not all subjects have Y values for
> the 3 conditions). Additionally, there is continuous measure W which was
> measured for all subjects.
>
> I am interested in testing the following:
>
> - Is the effect of W significant overall
> - Is the effect of W significant at each level of C
> - Is the effect of C significant
>
> In order to try to answer this, I have specified the following model with
> lmer:
>
> lmer( Y ~ W * C + (1 | ID), data = df)
>
> Which seems to proper reflect the structure of the data (I might be wrong
> here, any suggestions would be welcome).
> However when running the diagnostic plots I noticed a slope in the
> residuals plot and a slope different than y = x for the observed vs fitted
> plot (as shown bellow). Which made me question the validity of the model
> for inference.
>
> Could I still use this model for inference? Should I specify a different
> formula? Should I turn to lme and try to include different variances for
> each level of conditions (C)? Any ideas?
>
> I would be really appreciated if anyone could help me with this.
>
> Thanks in advance,
> Carlos Família
>
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