[R-meta] About choosing the reference

[Viechtbauer, Wolfgang (NP)]

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 using r-project.org> On Behalf
> Of ?????? via R-sig-meta-analysis
> Sent: Thursday, December 14, 2023 15:10
> To: r-sig-meta-analysis using r-project.org
> Cc: 英文科陳品誠 <t571 using 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
>
> 陳品誠 (Nick Chen)
> Email: t571 using wlgsh.tp.edu.tw <t5741 using wlgsh.tp.edu.tw>