[R-meta] Time as indicator vs time as meaning

[Stefanou Revesz]

Dear Wolfgang,

Thank you for your reply. The rma.mv() documentation for CAR says:
"the values of the "inner" variable should reflect the exact time
points of the measurement".

1) Does that mean I should use:  "time_meaning_wks | study" OR
"time_id | study"?

2) Can I have missing in "time_meaning_wks"?

3) Do you possibly have a demonstration showing how to interpret CAR
(or any other useful references to read about CAR)?

Thank you very much,
Stefanou

On Sat, Oct 9, 2021 at 7:52 AM Viechtbauer, Wolfgang (SP)
<wolfgang.viechtbauer using maastrichtuniversity.nl> wrote:
>
> Indeed. But then struct="CAR" would probably be more appropriate/parsimonious, since "UN" will estimate a different tau^2 for every unique week value and a different correlation for every possible pair of week values.
>
> Best,
> Wolfgang
>
> >-----Original Message-----
> >From: R-sig-meta-analysis [mailto:r-sig-meta-analysis-bounces using r-project.org] On
> >Behalf Of Michael Dewey
> >Sent: Saturday, 09 October, 2021 12:59
> >To: Stefanou Revesz; R meta
> >Subject: Re: [R-meta] Time as indicator vs time as meaning
> >
> >Dear Stefanou
> >
> >I think it would be find to use the continuous version both as fixed and
> >random effect.
> >
> >Michael
> >
> >On 09/10/2021 05:49, Stefanou Revesz wrote:
> >> Dear Meta-Analysis Colleagues,
> >>
> >> We are meta-analyzing 73 longitudinal studies. But we have doubts
> >> amongst us regarding how to combine the longitudinal effects of these
> >> studies.
> >>
> >> On the one hand, if we use time only as an indicator of testing
> >> occasions (pre-test and post-tests), and then use it as fixed and
> >> random-effect as in:
> >>
> >> rma.mv(es ~ time_id, random = ~ time_id | study, struct = "UN")
> >>
> >> then, we have longitudinally combined apples and oranges. That is,
> >> time 1 in one study may have covered six months, but time 1 in another
> >> study may have covered 6 days. This, we think, is problematic in terms
> >> of the interpretation of both the fixed and random-effects of time.
> >>
> >> So, we have coded for both time_id (testing occasions indicator) and
> >> time_meaning_wks (length of actual time up to each testing occasion in
> >> weeks).
> >>
> >> We are wondering how we should incorporate time_meaning_wks into our model?
> >>
> >> Any help is appreciated,
> >> Stefanou
> >>
> >> study  time_id   time_meaning_wks
> >> 1        0             0
> >> 1        1             4
> >> 1        2             6
> >> 2        0             0
> >> 2        1             1