Smooth spline modeling with brm()

Interfaces brms
1

I’m trying smooth spline modeling with brm(). However, it does not seem to allow me to handle smooth splines at the group level as indicated in the following error message:

Error: Cannot handle splines or GPs in group-level terms.

Is there any fundamental issue that does not permit group-level spline modeling? Or is it simply an implementation issue?

2

In general, please provide the code of the brms models you want to fit.

For your problem I fortunately know what you did so I can comment on your problem without seeing the code.

group-level splines have to be specified within the s() or t2() calls. I suggest you take a look a the doc of mgcv::s and look for “factor-smooth interactions” (argument bs="fs"). You will need to install the latest versions of mgcv and brms in order to run this. Be aware that such models may take very long time to fit.

1 Like
3

Thanks a lot, Paul!

One more question -

bf(Y ~ s(Age) + s(Age, by=factor1) + s(Age, by=factor2))

works fine. However, is there a way to do something like the following?

bf(Y ~ s(Age) + s(Age, by=factor1) + s(Age, by=factor2) + s(Age, by=factor1:factor2))

Currently it does not like this part: s(Age, by=factor1:factor2)

1 Like
4

I think you can do s(Age, by = interaction(factor1, factor2))

3 Likes
5

Thanks, Ben! Yes, that’s the trick.

6

G’day

I am very new to brms and R in general and would love some help on this issue.

I am trying to fit this brms model

fit_preference_nl ← brm(form_preference_nonlinear,
data = dfEUPA_NoNAs_SiteVisit_Single,
cores = cores,
chains = chains,
init_r = init_r,
iter = iter,
thin = thin,
seed = seed,
file = paste0(“results”,run_date),
control = control)

The formula I am using is

f_preference_linear ← cEatenUneatenPartiallyLikely ~ 1 + iAge +
cSex +
cFlavour+
iDaysSinceLastFed +
1|cIndividualID

and then added this code

form_preference_linear ← bf(f_preference_linear) + categorical()

When I try to fit the model, I the same error message as Gang:

Error: Cannot handle splines or GPs in group-level terms.

In paul.buerkner’s answer I saw he recommended looking into the factor-smooth interactions with argument bs=“fs”, however I am using bs=‘ts’ for my code, does this change the solution?

Thanks, Sean