[R-meta] robust variance estimation with small number of elements within the cluster

[Diego Grados Bedoya]

Dear all,

I would like to report the parameters obtained using a robust variance
estimation of a multilevel mixed model (including a categorical moderator)
without the intercept (sample size is 42). The categorical variable has 17
levels of which 7 of them only have 1 study (due to the nature of the
intervention). I am using as a cluster the study ID since some studies
contributed with more than 1 effect size. Using the clubSandwhich R
package, 4 out of the 7 levels of the moderator reported NA for the df and
CIs values. I think the reason is that the estimates of these levels
precisely coincide with the effect sizes obtained from the studies (se=0).
I wonder if someone has faced something similar? (the robust function from
metafor package is reporting the same values of the estimates).

Any hint is more than welcome,

Diego

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