[R-meta] Random effects structure
Viechtbauer, Wolfgang (SP)
wo||g@ng@v|echtb@uer @end|ng |rom m@@@tr|chtun|ver@|ty@n|
Wed Mar 25 15:43:58 CET 2020
Hi Cesar,
When the same group is used to compute multiple estimates (i.e., a common control), then this also induces dependency on the sampling errors. For the log response ratio, the covariance is then:
SD_C^2 / (n_C*M_C^2)
where M_C and SD_C are the mean and SD of the common control group and n_C the control group size.
So, you ideally should construct a proper V matrix that includes these covariances and that you can then pass to rma.mv().
But yes, random=~1|Site/exp/obs would be sensible.
Best,
Wolfgang
-----Original Message-----
From: R-sig-meta-analysis [mailto:r-sig-meta-analysis-bounces using r-project.org] On Behalf Of César Terrer
Sent: Wednesday, 25 March, 2020 15:34
To: r-sig-meta-analysis using r-project.org
Subject: [R-meta] Random effects structure
Dear community,
I am conducting a meta-analysis to study the growth rate of bacterial predators as compared to their prey, using the log response ratio. Furthermore, I want to study if this effect varies across different predators. The dataset has the following structure, here showing a subset:
Site CommonControl exp obs Predator lnR var
A Alaska 155 1 1 Bdello -0.6713152 0.03785708
A Alaska 155 1 2 Cytoph -0.0702467 0.05763364
A Alaska 155 1 3 Myxo -0.148982 0.00748768
A Alaska 1510 2 4 Bdello -0.4926361 0.01691187
A Alaska 1510 2 5 Cytoph -0.213787 0.01045785
B Andesite1controlWeek1 9 6 Bdello 0.27873598 0.14129722
B Andesite1controlWeek1 9 7 Cytoph -0.3243682 0.01466085
B Andesite1controlWeek1 9 8 Lyso 1.18302506 0.11663149
B Andesite1controlWeek6 11 9 Bdello -0.8465128 0.03701618
B Andesite1controlWeek6 11 10 Cytoph -0.1559056 0.0283173
B Andesite1controlWeek6 11 11 Lyso -0.8039415 0.04926915
1. There are different sites, thus a potential source of non-independency
2. Within each site, we use the value for preys in the denominator multiple times. I guess rows of data using the same denominator (CommonControl) are also potentially correlated and should be also added as a random-effect.
Based on 1., 2., and what I have understood from the Konstantopoulos (2011) tutorial, I think I should use the following model:
res <-rma.mv(yi=lnR, V=var, random=~1|Site/exp/obs, mods=~Predator, data=data)
Could you please let me know if the structure of random effects seems appropriate, and help me understand why I need to include "obs"?
Thank you.
Cesar
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