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

[Viechtbauer, Wolfgang (SP)]

To add to this:

2. Terms used in 'random' are not allowed to have missing values in rma.mv(), so those rows will need to be filtered out first before fitting the model.

3. rho in "CAR" is the autocorrelation for a one-unit difference in the time variable. So if time is measured in weeks, then rho reflects the correlation between two time points one week apart.

Best,
Wolfgang

>-----Original Message-----
>From: Michael Dewey [mailto:lists using dewey.myzen.co.uk]
>Sent: Saturday, 09 October, 2021 17:36
>To: Stefanou Revesz; Viechtbauer, Wolfgang (SP)
>Cc: R meta
>Subject: Re: [R-meta] Time as indicator vs time as meaning
>
>Comments in-line
>
>On 09/10/2021 15:56, Stefanou Revesz wrote:
>> 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"?
>
>Use the continuous one time_meaning_wks
>
>> 2) Can I have missing in "time_meaning_wks"?
>
>I assume it will work, just try it, nothing will break.
>
>> 3) Do you possibly have a demonstration showing how to interpret CAR
>> (or any other useful references to read about CAR)?
>
>If you type auto-regressive models into your favourite search engine you
>should find plenty of material. There are a couple of examples of AR
>models in the documentation, see ?rma.mv but neither of them is for a
>continuous covariate.
>
>> 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