Hi,

I’m new to bayesian modelling and brms. When specifiying a model for ordinal regression with multiple Likert items and IRT model they result in divergent transitions. Having trouble to find what is causing this issue. Any help on this topic is much appreciated.

Data generated using package LikertMakeR.

```
Sessioninfo
sessioninfo::session_info()
─ Session info ────────────────────────────────────────────────────────────────
setting value
version R version 4.4.1 (2024-06-14)
os macOS Sonoma 14.5
system aarch64, darwin20
ui RStudio
language (EN)
collate en_US.UTF-8
ctype en_US.UTF-8
tz Europe/Stockholm
date 2024-07-10
rstudio 2024.04.2+764 Chocolate Cosmos (desktop)
pandoc NA
─ Packages ────────────────────────────────────────────────────────────────────
package * version date (UTC) lib source
abind 1.4-5 2016-07-21 [1] CRAN (R 4.4.0)
backports 1.5.0 2024-05-23 [1] CRAN (R 4.4.0)
bayesplot 1.11.1 2024-02-15 [1] CRAN (R 4.4.0)
bridgesampling 1.1-2 2021-04-16 [1] CRAN (R 4.4.0)
brms 2.21.0 2024-03-20 [1] CRAN (R 4.4.0)
Brobdingnag 1.2-9 2022-10-19 [1] CRAN (R 4.4.0)
checkmate 2.3.1 2023-12-04 [1] CRAN (R 4.4.0)
cli 3.6.3 2024-06-21 [1] CRAN (R 4.4.0)
coda 0.19-4.1 2024-01-31 [1] CRAN (R 4.4.0)
codetools 0.2-20 2024-03-31 [1] CRAN (R 4.4.1)
colorspace 2.1-0 2023-01-23 [1] CRAN (R 4.4.0)
curl 5.2.1 2024-03-01 [1] CRAN (R 4.4.0)
distributional 0.4.0 2024-02-07 [1] CRAN (R 4.4.0)
dplyr 1.1.4 2023-11-17 [1] CRAN (R 4.4.0)
emmeans 1.10.3 2024-07-01 [1] CRAN (R 4.4.0)
estimability 1.5.1 2024-05-12 [1] CRAN (R 4.4.0)
fansi 1.0.6 2023-12-08 [1] CRAN (R 4.4.0)
generics 0.1.3 2022-07-05 [1] CRAN (R 4.4.0)
ggplot2 3.5.1 2024-04-23 [1] CRAN (R 4.4.0)
glue 1.7.0 2024-01-09 [1] CRAN (R 4.4.0)
gridExtra 2.3 2017-09-09 [1] CRAN (R 4.4.0)
gtable 0.3.5 2024-04-22 [1] CRAN (R 4.4.0)
hypr * 0.2.8 2023-11-09 [1] CRAN (R 4.4.0)
inline 0.3.19 2021-05-31 [1] CRAN (R 4.4.0)
jsonlite 1.8.8 2023-12-04 [1] CRAN (R 4.4.0)
lattice 0.22-6 2024-03-20 [1] CRAN (R 4.4.1)
lifecycle 1.0.4 2023-11-07 [1] CRAN (R 4.4.0)
loo 2.8.0 2024-07-03 [1] CRAN (R 4.4.0)
magrittr 2.0.3 2022-03-30 [1] CRAN (R 4.4.0)
MASS 7.3-60.2 2024-04-26 [1] CRAN (R 4.4.1)
Matrix 1.7-0 2024-04-26 [1] CRAN (R 4.4.1)
matrixStats 1.3.0 2024-04-11 [1] CRAN (R 4.4.0)
munsell 0.5.1 2024-04-01 [1] CRAN (R 4.4.0)
mvtnorm 1.2-5 2024-05-21 [1] CRAN (R 4.4.0)
nlme 3.1-164 2023-11-27 [1] CRAN (R 4.4.1)
pillar 1.9.0 2023-03-22 [1] CRAN (R 4.4.0)
pkgbuild 1.4.4 2024-03-17 [1] CRAN (R 4.4.0)
pkgconfig 2.0.3 2019-09-22 [1] CRAN (R 4.4.0)
posterior 1.6.0 2024-07-03 [1] CRAN (R 4.4.0)
pracma 2.4.4 2023-11-10 [1] CRAN (R 4.4.0)
QuickJSR 1.2.2 2024-06-07 [1] CRAN (R 4.4.0)
R6 2.5.1 2021-08-19 [1] CRAN (R 4.4.0)
Rcpp 1.0.12 2024-01-09 [1] CRAN (R 4.4.0)
RcppParallel 5.1.7 2023-02-27 [1] CRAN (R 4.4.0)
rlang 1.1.4 2024-06-04 [1] CRAN (R 4.4.0)
rstan 2.32.6 2024-03-05 [1] CRAN (R 4.4.0)
rstantools 2.4.0 2024-01-31 [1] CRAN (R 4.4.0)
rstudioapi 0.16.0 2024-03-24 [1] CRAN (R 4.4.0)
scales 1.3.0 2023-11-28 [1] CRAN (R 4.4.0)
sessioninfo 1.2.2 2021-12-06 [1] CRAN (R 4.4.0)
StanHeaders 2.32.9 2024-05-29 [1] CRAN (R 4.4.1)
stringi 1.8.4 2024-05-06 [1] CRAN (R 4.4.0)
stringr 1.5.1 2023-11-14 [1] CRAN (R 4.4.0)
tensorA 0.36.2.1 2023-12-13 [1] CRAN (R 4.4.0)
tibble 3.2.1 2023-03-20 [1] CRAN (R 4.4.0)
tidyselect 1.2.1 2024-03-11 [1] CRAN (R 4.4.0)
utf8 1.2.4 2023-10-22 [1] CRAN (R 4.4.0)
V8 4.4.2 2024-02-15 [1] CRAN (R 4.4.0)
vctrs 0.6.5 2023-12-01 [1] CRAN (R 4.4.0)
xtable 1.8-4 2019-04-21 [1] CRAN (R 4.4.0)
[1] /Library/Frameworks/R.framework/Versions/4.4-arm64/Resources/library
───────────────────────────────────────────────────────────────────────────────
```

```
#Model 1
fit_cu2 <- brm(
formula = Loy ~ 1 + cs(Country),
data = rdatLong,
family = acat("probit")
)
#Model 2 with priors from Bayesian Item Response Modeling in R with brms and Stan by Bürkner
irt1PL <- bf(Loy ~ 1 + (1 | Item) + (1 | Ind))
prior_irt1PL <-
prior("normal(0, 3)", class = "sd", group = "Ind") +
prior("normal(0, 3)", class = "sd", group = "Item")
fit_irt1PL <- brm(
formula = irt1PL,
data = rdatLong2,
family = brmsfamily("cumulative", "logit"),
prior = prior_irt1PL
)
```

Code for simulated data

```
C1 <- lfast(
n = 25,
mean = 6,
sd = 1.0,
lowerbound = 1,
upperbound = 7,
items = 1
)
C2 <- lfast(
n = 25,
mean = 6,
sd = 1.0,
lowerbound = 1,
upperbound = 7,
items = 1
)
C3 <- lfast(
n = 25,
mean = 4,
sd = 2.0,
lowerbound = 1,
upperbound = 7,
items = 1
)
C4 <- lfast(
n = 25,
mean = 3,
sd = 2.0,
lowerbound = 1,
upperbound = 7,
items = 1
)
```

The data is compiled to long format before trying to fit the model.