RE Variance estimate zero

[florianhartig]

https://stats.stackexchange.com/questions/115090/why-do-i-get-zero-variance-of-a-random-effect-in-my-mixed-model-despite-some-va

https://stats.stackexchange.com/questions/34969/random-effect-equal-to-0-in-generalized-linear-mixed-model/34979#34979

https://stat.ethz.ch/pipermail/r-sig-mixed-models/2014q3/022509.html
https://discourse.mc-stan.org/t/maximum-likelihood-estimation-for-random-effect-model-for-meta-analysis/37394/9

Chung, Y., Rabe‐Hesketh, S., & Choi, I. H. (2013). Avoiding zero between‐study variance estimates in random‐effects meta‐analysis. Statistics in medicine, 32(23), 4071-4089.