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

[Viechtbauer, Wolfgang (SP)]

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