Wide CIs in last level when using "contr.sum"

When using sum contrasts for factors, the conditional effect (from conditional_effects()) of the last level in the factor always has a particularly wide credible interval (see figure for an example). This happens independent of data and model.

Can someone shed light on this? (Might be related to this, but I am not sure: Extremely wide 95% CIs when using custom contrast coding)

[brms 2.12.0, R 3.6.2, macOS 10.15.4]

Hi @stefanoc88.

Welcome to the forum! This is an interesting question and it would be helpful if you provided more information on the data you are working with or a reproducible example (e.g. simulation). I tried to reproduce this with a small simulation on my own system (windows 10, R 3.6.2 brms 2.12.0) but could not reproduce this ( find the example here: example_contr_sum.R (4.1 KB) ).
As you can see in the plot below, if simulated equally large the different CIs should indeed also be equally large with sum-to-zero coding.

.

Can you provide more information about the data and models you are working with?

Best,
Julian

3 Likes