Hi everyone!

New to the forum, first brms model.

I am triying to fit a multilevel model with random intercepts + slopes, and I am not sure how to add the splines to random efects in brms.

model<- brm(

bf( y ~ +Time + gender+ageonset +t x + Time:gender+ Time:ageonset+

(Time | ID)),

data = dat,

family = “gaussian”,

prior = c(

prior(normal( 0, 1), class = “Intercept”),

prior(normal( 1, 1), class = “b”, coef = “Timestd” ),

prior(normal(0, 1), class = “b”),

prior(exponential(1), class = “sd”),

prior(lkj(1), class = “cor”),

prior(exponential(1), class = “sigma”)),

control = list(adapt_delta = .95),

iter = 2000, warmup = 1000, chains = 4, cores = 4,

seed = 1215)

I read in previous post you can add s(Time, ageonset, bs = “cr”, k = 10) + s(Time, gender, bs = “cr”, k = 10) but do not know how to add that to the random effects (Time | ID).

Thanks for your help.

PS = is there something about strategies for selecting priors for splines?