[R-meta] Network meta-analysis: including a within-person (split-leg) study + pooling absolute vs relative changeNetwork meta-analysis: including a within-person (split-leg) study + pooling absolute vs relative change

[Viechtbauer, Wolfgang (NP)]

How the data needs to be organized depends on the software you are using. If you are using rma.mv() from metafor, then it would indeed need to be in long format (for all studies), so:

Study trt  mean
I     A    100
I     B    150
II    A    200
II    C    200
...
IV    B    170
IV    C    180

and the V matrix is diagonal, except for the 2x2 block from study IV, where there is also the covariance.

Best,
Wolfgang

> -----Original Message-----
> From: Jorge Teixeira <jorgemmtteixeira using gmail.com>
> Sent: Wednesday, October 28, 2026 13:22
> To: Viechtbauer, Wolfgang (NP) <wolfgang.viechtbauer using maastrichtuniversity.nl>
> Cc: R Special Interest Group for Meta-Analysis <r-sig-meta-analysis using r-
> project.org>
> Subject: Re: [R-meta] Network meta-analysis: including a within-person (split-
> leg) study + pooling absolute vs relative changeNetwork meta-analysis: including
> a within-person (split-leg) study + pooling absolute vs relative change
> 
> Thanks Wolfgang.
> 
> I did that to highlight that the means were not independent.
> 
> I also assumed that the data had to be in long format to estimate the
> V matrix. Is that not the case? :)
> 
> On Wed, 28 Oct 2026 at 10:45, Viechtbauer, Wolfgang (NP)
> <wolfgang.viechtbauer using maastrichtuniversity.nl> wrote:
> >
> > Hi Jorge,
> >
> > Much more readable!
> >
> > I see no inherent problem with including study IV, but not sure why you
> structured it as you did. In essence, it is like this:
> >
> >            Obs     Tx A    Tx B    Tx C
> > Study I      1       100     150       -
> > Study II     1       200       -     200
> > Study III    1       200     200       -
> > Study IV     1         -     170     180
> >
> > But since study IV is a within-subjects design, the two means are not
> independent. So you would have to compute the covariance between them and
> include this in the V matrix. The covariance between the two means is a function
> of the correlation of the raw measurements for the two legs. If this is not
> reported, then you could use a guestimate and do a sensitivity analysis.
> >
> > Best,
> > Wolfgang
> >
> > > -----Original Message-----
> > > From: R-sig-meta-analysis <r-sig-meta-analysis-bounces using r-project.org> On
> Behalf
> > > Of Jorge Teixeira via R-sig-meta-analysis
> > > Sent: Wednesday, October 28, 2026 10:57
> > > To: Michael Dewey <lists using dewey.myzen.co.uk>; R meta <r-sig-meta-analysis using r-
> > > project.org>
> > > Cc: Jorge Teixeira <jorgemmtteixeira using gmail.com>
> > > Subject: Re: [R-meta] Network meta-analysis: including a within-person
> (split-
> > > leg) study + pooling absolute vs relative changeNetwork meta-analysis:
> including
> > > a within-person (split-leg) study + pooling absolute vs relative change
> > >
> > > Thanks, Michael. And now?!
> > >
> > > I'm planning a small network meta-analysis and would value your
> > > input on two design questions.
> > >
> > > SETUP
> > >
> > > I have three parallel-group RCTs comparing treatments A, B and C
> > > (mean value at post per arm):
> > >
> > >               Tx A    Tx B    Tx C
> > >   Study I      100     150       -
> > >   Study II     200    200      -
> > >   Study III      -     200     200
> > >
> > > I would like to add a fourth study (Study IV). It is a within-person
> > > design: each participant's legs were randomized to different
> > > treatments, so comparisons are within-group rather than
> > > between-group. I have the post-treatment means + SD for each
> > > condition.
> > >
> > > QUESTION
> > >
> > > Can Study IV be meta-analyzed together with the three parallel RCTs?
> > >
> > >             Obs     Tx A    Tx B    Tx C
> > >   Study I      1       100     150       -
> > >   Study II     1       200       -     200
> > >   Study III    1       200     200       -
> > >   Study IV     1         -       -     180
> > >   Study IV     2         -     170       -
> > >
> > > Thanks a lot for your help.
> > >
> > > Best,
> > > Jorge
> > >
> > > On Wed, 28 Oct 2026 at 09:12, Michael Dewey <lists using dewey.myzen.co.uk> wrote:
> > > >
> > > > Dear Jorge
> > > >
> > > > You have set your mailer to post in HTML which mean that we are not
> > > > seeing what you posted. Not sure how you get plain text.
> > > >
> > > > I think the answer to question 2 is No. It is fairly easy to create
> > > > examples with the same absolute change and different relative change.
> > > >
> > > > Michael