In brms ordinal models the cumulative family only supports the thres() addition term, but does not support cs (category-specific effects). The cs terms are available only for the acat, cratio and sratio ordinal families. (this follow from how the various forms of ordinal models get defined - see the tutorial by Paul for a good treatment: SAGE Journals: Your gateway to world-class journal research)
Additionally, I think the syntax for a varying category-specific intercept should be cs(country | item)
It appears that brms should have given you a more informative error.
So you either need to have something like | thres(gr=item_country) where item_country is the interaction of item and country or switch to cratio (or other) model and have something like cs((country | item) + item) (I didn’t test this precisely, so if you encounter additional difficulties, feel free to ask for more clarifications).
Hi, thank you for your reply. I am using acat ordinal family, sorry for not specifying that.
I have used your idea of | thres(gr=item_country) and it provides me an intercept for each country and items have different thresholds as aspected.
However, the cs(country | item) is not working, maybe still not implemented this change according to a previous post of Paul: Not with the cumulative family due to the order requirements of the thresholds. Other ordinal families support that via adding a (cs(1) | item) . Note that I don’t like the current category specific (cs) syntax for varying effects and might change them to cs(1 | item) at some point.
I see a few problems - thres is AFAIK not supported for an acat model. Additionally as per help for brmsformula, a category-specific varying intercept should be specified as (cs(country) | item), i.e. the grouping should not be part of the cs term. Unfortunately the error message from brms is not very informative in this case.