Message-ID: <AS8PR08MB91939237BE3934A67A5910338B8CA@AS8PR08MB9193.eurprd08.prod.outlook.com>
Date: 2023-12-14T16:11:34Z
From: Wolfgang Viechtbauer
Subject: [R-meta] About choosing the reference
In-Reply-To: <CAPm5Ok2S1sn9kuu+DaJJrA+RJJtX29J-w-4=Ec6V=AYwM5kNZQ@mail.gmail.com>
Dear Nick,
Please do not post in HTML format. See what this can do to posts -- makes things quite unreadable. See also the notes on the mailing list page: https://stat.ethz.ch/mailman/listinfo/r-sig-meta-analysis
All of these results just reflect different contrasts of the same two-way interaction. So there is not one that is inherently preferable. This is not an issue specific to meta-analysis. You would face the same issue in any regression model with such a two-way interaction. If the two-way interaction as a whole is significant (based on a test of all three interaction terms, so the chi^2- or F-test with df=3), then one could examine four simple contrasts (between M and U of pos for each of the four levels of dur).
Best,
Wolfgang
> -----Original Message-----
> From: R-sig-meta-analysis <r-sig-meta-analysis-bounces at r-project.org> On Behalf
> Of ?????? via R-sig-meta-analysis
> Sent: Thursday, December 14, 2023 15:10
> To: r-sig-meta-analysis at r-project.org
> Cc: ?????? <t571 at wlgsh.tp.edu.tw>
> Subject: [R-meta] About choosing the reference
>
> Dear all,
>
> When performing meta-regression analysis, I use this code to change
> turn a variable into a factor:
>
> dat$duration <- relevel
> <http://stat.ethz.ch/R-manual/R-devel/library/stats/html/relevel.html>(factor
> <http://stat.ethz.ch/R-manual/R-
> devel/library/base/html/factor.html>(dat$duration),
> ref="S")
>
> In my data, the duration moderator has four values including S
> (short), M (medium), L (long), and NA (not provided). And I tried to figure
> out the interaction between this moderator and another moderator named
> "Posttest type" with two values of M (meaning) and U (use). The problem I
> face now is that with different ference I used, the coefficiency will
> differ, for example,
> (a) I use M as reference for duration, and this is the result
>
> estimate se zval pval
> ci.lb ci.ub
>
> durS:posU 2.3309 0.8690 2.6824 0.0073 0.6278 4.0341 **
>
>
> (b) I use N/A as reference for duration, and this is the result
>
> estimate
> se zval pval ci.lb ci.ub
>
> durS:posU 1.7890 1.4294 1.2516 0.2107 -1.0126 4.5906
>
> (c) I use L as reference for duration, and this is the result
>
> estimate se zval
> pval ci.lb ci.ub
>
> durS:posU 1.2827 0.9275 1.3829 0.1667 -0.5352 3.1005
>
>
> So, eventually, which one should I choose to be the final result? Is
> there any criteria to choose between the three?
>
> Nick
>
> ??? (Nick Chen)
> Email: t571 at wlgsh.tp.edu.tw <t5741 at wlgsh.tp.edu.tw>