[R] Syntax differences between aov and lmer for 2-way repeated measures design using a mixed model

[Bert Gunter]

You should talk with your professor.  This list is about R programming.
Essentially statistical issues, which this appears mostly to be, are
generally off topic.

Questions about mixed effects models -- RM and longitudinal designs are
typically analysed as such -- and especially using the nlme and/or lme4
packages are usually better posted on the r-sig-mixed-models list.

... and if this is homework, this list has a no homework poilicy.

Cheers,

Bert Gunter

"The trouble with having an open mind is that people keep coming along and
sticking things into it."
-- Opus (aka Berkeley Breathed in his "Bloom County" comic strip )


On Fri, Apr 12, 2019 at 11:08 AM Uri Eduardo Ramírez Pasos <
urieduardo using gmail.com> wrote:

> Hi everyone,
>
> I'm working with the following data frame using R. It consists of
> measurements obtained from 7 subjects with two independent variables (IV1
> and IV2) with two levels each (OFF/ON, ALT/ISO, respectively):
>
> >myData
> Subject      DV         IV1     IV2
>         1   2.567839      OFF      ALT
>         1  58.708027       ON      ALT
>         1  44.504265      OFF      ISO
>         1 109.555701       ON      ISO
>         2  99.043735      OFF      ALT
>         2  75.958737       ON      ALT
>         2 182.727396      OFF      ISO
>         2 364.725795       ON      ISO
>         3  45.788988      OFF      ALT
>         3  52.941263       ON      ALT
>         3  54.719013      OFF      ISO
>         3  41.909909       ON      ISO
>         4 116.145279      OFF      ALT
>         4 162.927971       ON      ALT
>         4  34.162077      OFF      ISO
>         4  74.029748       ON      ISO
>         5 114.412913      OFF      ALT
>         5 121.127983       ON      ALT
>         5 192.379708      OFF      ISO
>         5 229.192453       ON      ISO
>         6 213.421076      OFF      ALT
>         6 526.739206       ON      ALT
>         6 150.596812      OFF      ISO
>         6 217.931951       ON      ISO
>         7 117.931273      OFF      ALT
>         7 102.467813       ON      ALT
>         7  57.823062      OFF      ISO
>         7  85.181033       ON      ISO
> (1) Is this a repeated measures (RM) design? Some folks have mentioned that
> it is not since it isn't a longitudinal study, but I thought that as long
> as there are measurements from each experimental unit for every single
> level of a factor, one can say this as a RM design. What is correct? Also,
> is an RM design synonymous with having a within-subject factor?
>
> (2) I'm interested in both the main and the interaction effects of IV1 and
> IV2, but due to having measurements from each subject for all level
> combinations, I think I have to include Subject as a random effect. I have
> looked at aov and lmer but I'm confused about the difference in syntax:
> This cheat sheet recommends:
>
> m1<-aov(DV ~ IV1*IV2 + Error(Subject/(IV1*IV2)), myData)
>
> However it's not clear to me whether Error(x/(y*z)) means x is a random
> effect and y and z are nested in x. Is this interpretation correct? If so,
> would m1 be inappropriate for my data since my data isn't nested, but fully
> crossed? And if so, would
>
> m2<-aov(DV ~ IV1*IV2 + Error(Subject), myData)
>
> be the correct syntax? I have also been told that in m2 the Error term
> should be dropped - is this correct?
>
> (3) In a previous question I was told the linear mixed effects model
>
> m3<-lmer(DV ~ IV1*IV2 + (1|Subject), myData)
> was appropriate more my data. Just to better understand lmer syntax: if I
> had n subjects and for each subject measurements were obtained for both
> levels of IV2 but half of the subjects were OFF and the other half ON,
> would the model be
>
> m4<-lmer(DV ~ IV1*IV2 +(1|Subject/IV1), data=myData) ?
>
> And if there was only one measurement per IV1*IV2 combination, would that
> mean this is no longer a repeated-measures design and therefore the model
> is just
>
> m5<-lmer(DV ~ IV1*IV2, data=myData) ? In which case lm would probably
> suffice.
>
> Any help would be greatly appreciated,
> Uri Ramirez
>
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>
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