[R-meta] guidance for modeling SMCC type effect size

[Reza Norouzian]

Dear Stefanou,

The modeling functions like rma.mv(), rma.uni() etc. don't generally
depend on the type of effect size. That aside, just to make sure, did
you compute the change from each pre-test to each follow-up post-test,
or the change from each testing occasion to the following one (e.g.,
pre-test to post-test1, post-test1 to post-test2 ...)?

Regardless of how you define the intervals, if your studies include a
control (C) and multiple treatment conditions (T1, T2,...) that occur
across the studies, you can create a 'condition' variable to
distinguish between the control and the treatments' SMCCs (yi) in each
study over the intervals you have defined:

study condition  yi  interval_id id
1     T1        .1   0           1
1     T1        .3   1           2
1     T2        .7   0           3
1     T2        .2   1           4
1     C         .4   0           5
1     C         .5   1           6
2     T2        .6   0           7
2     C         .9   1           8

In which case, a starting point might be:

rma.mv(yi ~ condition*interval_id, V = Some_V_matrix, random = list(~
interval_id | study, ~ 1 | id), struct = "HAR")

This model assumes that in each study, true SMCCs at all intervals are
auto-regressively correlated with each other regardless of the
conditions they belong to. If in each study, true SMCCs at all
intervals across the conditions are assumed to have their own
auto-regressively correlated structure as well, then, you can
consider:

rma.mv(yi ~ condition*interval_id, V = Some_V_matrix, random = list(~
interval_id | study, ~ interval_id | interaction(study,condition), ~ 1
| id), struct = c("HAR","HAR") )

In both models, when the interaction term is decomposed to its simple
effects for each condition, you get the average SMCC for each
condition (e.g., T1, T2, or C) across the intervals for your studies.

For your second type of effect size (y_i = SMCC_T - SMCC_C, v_i =
v_{i_{SMCC_T}} + v_{i_{SMCC_C}}), pretty much everything I said above
applies. However, in this case, you're basically modeling some sort of
simple effect for each treatment's change vs the control's change
across the intervals for your studies. So, by fitting the above model
but using this type of effect size, you'll be asking how such simple
effects change over the interval you have considered.

I believe these metrics for effect size are nowadays less commonly
used, partly because you can use an SMD metric, which among other
things doesn't require direct knowledge of pre-post correlations for
their computation, model them using multivariate-multilevel models,
and present the results in perhaps more intuitive ways to your
audience.

Does that help?

Reza



Reza


On Sun, Sep 12, 2021 at 6:15 AM Stefanou Revesz
<stefanourevesz using gmail.com> wrote:
>
>
> Dear Experts,
>
> I need some guidance for modeling two types of effect sizes (for two
> different meta-analyses) using the rma.mv() program in metafor.
>
> First, I have computed the standardized mean change (SMCC) between
> each pre-test and the follow-up post-tests in each of
> multiple-treatment studies.
>
> Second, I have computed the difference in standardized mean changes
> (SMCC) between a treatment and a control group at each pre-test to
> post-test intervals in each of multiple-treatment studies.
>
> The reason I want to use rma.mv() is that each of my studies could
> produce multiple of these types of effect sizes. But I wonder what
> would be a starting point for modeling these two types of effect size?
>
> Any help is appreciated,
> Stefanou
>
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