Describe the brms summaries of `recover_exchangeable_covariance()` accurately
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Assessment
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- 1-3 hours
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- Documentation
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- Tech stack
- r
- Domain
- documentation
Research direction
Read the help entry for recover_exchangeable_covariance() and compare it with the brms summary logic in R/model_exchangeable_covariance.R (lines 282–290). Update the return-value description to say that brms summaries of varcov and sdcor are separate and need not agree, and that medians are taken element by element.
Written by the indexing model from the issue text.
Description
For brms models with posterior = "mean" or "median", the function summarises varcov and sdcor separately, element by element. Two consequences are missing from the help:
sdcorholds the mean (or median) of the SD and correlation draws. It is not the SD and correlation form of the returnedvarcov, and the two cannot be converted into each other. In a small fit during the review (20 dyads), the partner correlation was 0.57 insdcorand 0.59 when computed fromvarcov.- The median is taken element by element. The resulting matrix need not be a valid covariance matrix.
Both are small next to the posterior uncertainty. The help calls sdcor the SD and correlation representation of varcov, which is only exact for glmmTMB and for posterior = "draws".
Suggested fix and checks
- Add one sentence to the return value: for brms summaries,
varcovandsdcorare summarised separately and need not agree. - Say that the median is element-wise.
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