BRMS Wiener model with non-logged vs logged RT

Interfaces brms
1

(I have looked at previous questions about this same issues but the suggestions don’t seem to work anymore or I still want to understand what is happening)

When fitting a model using a Wiener family, I get the “Initial value rejected” error before sampling starts. If I use log-transformed values, then sampling starts fine. Here’s the code

library(tidyverse)
shallow <- read_csv("https://github.com/uoelel/qml-data/raw/main/data/song2020/shallow.csv") |> 
  filter(
    Critical_Filler == "Critical",
    RT > 0
  )

wm1_bf <- bf(
  # RTs are in ms
  RT | dec(ACC) ~ Relation_type * Group + (Relation_type |p| ID),
  bs ~ Group + (1 |p| ID),
  ndt ~ Group + (1 |p| ID),
  bias ~ Group + (1 |p| ID)
)

wm1 <- brm(
  wm1_bf,
  family = wiener(),
  data = shallow_filt,
  seed = my_seed,
  backend = "cmdstanr",
  chain = 1
)

I understand this is related to the fact that brms picks initial values in a particular way but I don’t quite understand it completely so I am not sure why including logged RT instead of RT in ms solves the issue.

  • Operating System: macOS Sonoma 14.1
  • brms Version: 2.21.0
1 Like
2

I’m not familiar with reaction time models, but I sometimes run into this problem when the (default) priors are too weak. I suggest setting weakly informative priors. I naively pulled the priors from this post, and I was able to get the model through initialization to start sampling.

3

I also don’t have experience with the Wiener family, but as a general strategy when you have start values rejected, you might try setting init_r to some value lower than 2, something like init_r = 0.2.

4

I’ve seen that argument in Paul’s tutorial on IRT models, but it doesn’t seem to be available anymore. I couldn’t find it neither in the brm() nor in the cmdstanr help. :(

5

It appears to be available in version 2.20.4. I can’t say what’s happened since.

6

The init_r argument also still seems to be part of one of Paul’s code tests: brms/tests/local/tests.models-3.R at 98df56deef049e1e70d89e66876092179a7ca2d8 · paul-buerkner/brms · GitHub