[R-meta] Moderation analysis in IPD meta-analysis

Dear Corentin,

Overall, your approach seems sound. But a few notes:

1) study1<-glm(DV~IV, data=datastudy1) is not logistic regression. You
need:

study1 <- glm(DV ~ IV, data=datastudy1, family=binomial)

2) I've only played around with emmeans a little bit, so can't comment on
that part. But I don't think you even need it. You can just fit the model
in such a way that you directly get the three log odds ratios for the three
levels of IV. In fact, the estimates of the three log odds ratios are
independent, so one could even just fit three simple logistic regression
models that will give you the same results. An example:

dat <- data.frame(DV = c(1,0,0,1,0,1,1,1,0,1,1,1),
                  IV = c(1,3,2,3,5,3,7,7,4,9,6,3),
                  VM = rep(c("a","b","c"),each=4))

# parameterize logistic regression model so we get the three log odds
ratios directly
res <- glm(DV ~ VM + IV:VM - 1, data=dat, family=binomial)
summary(res)

# the covariance between the three estimates is 0
round(vcov(res), 5)

# show that the simple logistic regression model for a subset gives the
same results
res.a <- glm(DV ~ IV, data=dat, family=binomial, subset=VM=="a")
summary(res.a)

So actually the V matrix corresponding to the three log odds ratios is
diagonal. But you still would want to account for potential dependency in
the underlying true log odds ratios, so the model

model <- rma.mv(yi, V, mods = ~ VM-1, random=~VM|study, struct="UN",
data=dat)

is still appropriate (with V being diagonal, so you can also just pass a
vector with the sampling variances to rma.mv).

3) The statement that 2-stage approaches cannot be used to analyze
patient-level moderators isn't quite true. If one actually analyzes the
patient-level moderator in stage 1 (as you describe), then the 2-stage
approach definitely allows you to examine such a patient-level moderator.

Best,
Wolfgang

-----Original Message-----
From: R-sig-meta-analysis [mailto:

r-sig-meta-analysis-bounces at r-project.org]

On Behalf Of GOSLING Corentin
Sent: Friday, 07 August, 2020 20:03
To: r-sig-meta-analysis at r-project.org
Subject: [R-meta] Moderation analysis in IPD meta-analysis

Dear Metafor?s users,

This is the first time that I post on this mailing list.

Our team is currently planning an individual patient data meta-analysis of
prospective cohorts.

We adopt an IPD approach because no prospective study has yet reported

this

association while many should have the data to assess it (i.e., they have
access to the targeted variables but did not report the association).
Unfortunately, we anticipate that most of the included studies will not
have an ethics committee that gives the right to share their data with us.

To overcome this, we plan to ask the authors of the included studies to
perform the analyses on their own data and to share only the results of

the

analyses with us. Our dependent variable is binary (DV: yes/no) and our
independent variable is an ordered variable (IV: a scale variable in 12
points [from 1 to 12]) treated as a continuous variable.

We ask authors to perform a logistic regression. Based on their results
(log odds ratio and associated variance), we adopt a classic two-stage
approach. I show the R code for a particular study to highlight our
approach.

#R code for study 1

study1<-glm(DV~IV, data=datastudy1)

yi1<- summary(study1)$coefficients[2,1] #extract the log odds ratio

vi1<- summary(study1)$coefficients[2,2]^2 #extract the squared standard
error

# then, we repeat the same process for each included study

# Once all the effect sizes and their variance are collected, we can store
them within a dataset and run a standard two stage meta-analysis

dat<-data.frame(
yi=c(yi1, yi2, yi3?),
vi=c(vi1, vi2, vi3?),
study=c(1, 2, 3?))

model<-rma(yi,vi, dat)

This is the code for our primary analysis. In an exploratory analysis, we
would like to perform a moderation analysis with a patient-level

moderator.

We understand how to perform a moderation analysis for a study-level
moderator but we are not sure on how to implement it with a patient-level
moderator. The aim of this moderation analysis will be to obtain the
estimated average effects for each level of a moderator. I describe here
the approach we have envisaged:

# example of R code for study 1

#let VM denote a participant-level moderator with 3 categories (a,b,c)

study1<-glm(DV~IV*VM, data=datastudy1)

EM1<-emmeans::emtrends(study1, ~VM, var=" IV")

yi1<- as.data.frame(EM1$emtrends)[,2] #extract log odds ratio for each
level of the VM for study 1 (contains 3 values)

V1<-vcov(EM1) # extract the variance/covariance matrix for study 1 (a 3x3
matrix)

# then, we can build a dataset which will look like this...

dat<-data.frame(
yi=c(yi1, yi2, yi3?),
VM=c(a,b,c,a,b,c,a,b,c?),
study=c(1,1,1, 2,2,2, 3,3,3?))

# ...and a variance-covariance matrix using the bldiag function

V<-bldiag(list(V1,V2,V3?))

# Last, we plan to perform a multivariate model in which we leave out the
model intercept and in which we use an unstructured variance structure (if
the model converges).

model<-rma.mv(yi, V, mods = ~ VM-1, random=~VM|study, struct="UN",

data=dat)

We were wondering if you could give us some feedback on the correctness of
our approach. We have read in several textbooks that two-stage
meta-analyses are not designed to assess patient-level moderator but,

given

that asking for raw data would probably decrease the likelihood of getting
return from authors of primary studies, we would prefer staying at a
two-stage approach.

Thank you very much for your help and for this amazing mailing list!

Corentin J Gosling
Charlotte Pinabiaux
Serge Caparos
Richard Delorme
Samuele Cortese