[R-sig-ME] Repeated Measures Design With lme Function
Thierry Onkelinx
thierry.onkelinx at inbo.be
Mon Feb 12 17:18:38 CET 2018
Dear Telli,
When you nest question in category, you assume that you have 25 (= 5 x
5) different question which are grouped into 5 categories.
In lme4 notation the random effects (1|SubjectID/Category/Question)
translate more verbose into (1|SubjectID) + (1|SubjectID:Category) +
(1|SubjectID:Category:Question). This make it more clear that you have
3 random intercepts: one per SubjectID, one per combination of
SubjectID and Category and one per combination of SubjectID, Category
and Question. That last random intercept has only one observation per
level and thus doesn't make sense. So I'd go for lme(Essen ~
AgeinYears * Category * Question, random = ~1|SubjectID/Category) Note
that I've rearraged the order of the fixed effects. This is yield a
different parametrisation which seems a bit more appropriate for this
case.
Best regards,
ir. Thierry Onkelinx
Statisticus / Statistician
Vlaamse Overheid / Government of Flanders
INSTITUUT VOOR NATUUR- EN BOSONDERZOEK / RESEARCH INSTITUTE FOR NATURE
AND FOREST
Team Biometrie & Kwaliteitszorg / Team Biometrics & Quality Assurance
thierry.onkelinx at inbo.be
Havenlaan 88 bus 73, 1000 Brussel
www.inbo.be
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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
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2018-02-10 21:59 GMT+01:00 Telli Davoodi <telli at bu.edu>:
> Hi all,
>
> I'm having a hard time defining a model with my repeated measures design
> (explained below). This is an experimental design and it is fully balanced.
> I would really appreciate it if you have any feedback.
>
> Thanks,
> Telli
>
> Here's the design of my experiment: I have 72 children (Subject ID) answer
> five questions (Question) about five different categories (Category).
> Basically, the same five questions repeat for each category. So, in long
> format, every participant has 25 rows (five for each level of category and
> within each level of category, question has five levels).
>
> Now, I want to fit a mixed-effects regression model on this data, using the
> lme function from the nmle package, but I'm not sure how to account for the
> fact that Category repeats with Subject ID and Question repeats with
> Category. This is what I have so far, but I'm not sure if I'm specifying
> the random part of the model correctly:
>
> summary(Qs <- lme(Essen ~ Question * AgeinYears * Category,
> random = ~1|SubjectID/Category/Question, data = Q, na.action = na.omit))
>
> I just want to make sure that I am allowing Category to vary within
> SubjectID and Question to vary within Category.
>
> Also, is it correct to say that Category is "nested" under Subject ID and
> Question is nested under Category?
>
> [[alternative HTML version deleted]]
>
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