Syntax error with brms hierarquical non-linear model

Interfaces brms
1

Dear Stan/brms community,

I am currently having trouble when mixing multilevel with nonlinear models.
I want to introduce harmonic terms (sine wave) in the model. The following works:

pois_form <- bf(y ~ a*sin(b*t) + sex, a+b~1+sex,nl=T)
ab_priors <- prior(cauchy(0, 1), nlpar = "a", lb = 0) +
  prior(cauchy(0, 1), nlpar = "b",lb = 0)
poiss_fit <- brm(pois_form, prior = ab_priors,
                 data = gp_df, family = poisson())

However, when introducing a random effects term (either with (1 |gr(ParticipantID)) or (1 |ParticipantID))), I get a parsing error which seems to be related to the multilevel term in the Stan output code:

My formula becomes:

pois_form <- bf(y ~ a*sin(b*t) + sex + (1 |gr(ParticipantID)), a+b~1+sex,nl=T)

Which returns the follwowing error

SYNTAX ERROR, MESSAGE(S) FROM PARSER:
 error in 'model24a9763592c2_file24a96eaf1d9c' at line 36, column 59
  -------------------------------------------------
    34:   for (n in 1:N) {
    35:     // compute non-linear predictor values
    36:     mu[n] = nlp_a[n] * sin(nlp_b[n] * C_1[n]) + C_2[n] + (1|gr(C_3[n]));
                                                                  ^
    37:   }
  -------------------------------------------------
PARSER EXPECTED: ")"
Error in stanc(model_code = paste(program, collapse = "\n"), model_name = model_cppname,  : 
  failed to parse Stan model 'file24a96eaf1d9c' due to the above error.

Am I missing something? Any ideas on how to fix it? I would also welcome any material on spectral decomposition (Fourier terms) for multilevel modeling considering covariates.

2

Welcome to the Stan forums! This question has been asked in differnt form in multiple threads already. Essentially, a formula can even be linear or non-linear with respect to how it is parsed. Multilevel terms only work in linear formulas, in your case in formulas for a and b not in the formula of y, which is nonlinear.

1 Like
3

Thank you, Paul.

I suppose I’ll have to work from make_stancode then.