[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,

Can you show us a plot of the residuals versus W for each level of C? It
looks like either the relation of Y and W is not linear, or you are missing
an important covariate.

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

2016-10-03 10:34 GMT+02:00 Carlos Familia <carlosfamilia op gmail.com>:

> Hello,
>
> The image can be found here https://s18.postimg.org/
> rbx2vh2ex/Pasted_Graphic_4.png
>
> Best regards,
> Carlos Família
>
> On 3 Oct 2016, at 08:50, Thierry Onkelinx <thierry.onkelinx op inbo.be>
> wrote:
>
> Dear Carlos,
>
> Your plot got stripped from your mail. Try sending it as pdf or put it
> someone online and send us the URL.
>
> 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
>>
>> _______________________________________________
>> R-sig-mixed-models op r-project.org mailing list
>> https://stat.ethz.ch/mailman/listinfo/r-sig-mixed-models
>
>
>

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