I am having trouble finding the correspondance between gamm4 and stan_gamm4.

For example, if I fit a simple gam model with gamm4, I get the following output:

```
library(tidyverse)
library(rstanarm)
gamm4(Sepal.Width ~ Sepal.Length + s(Petal.Width), random=~(1|Species), data=iris) %>%
.$gam %>%
summary()
```

```
Family: gaussian
Link function: identity
Formula:
Sepal.Width ~ Sepal.Length + s(Petal.Width)
Parametric coefficients:
Estimate Std. Error t value Pr(>|t|)
(Intercept) 1.46144 0.61708 2.368 0.0192 *
Sepal.Length 0.27311 0.04672 5.846 3.13e-08 ***
---
Signif. codes: 0 ‘***’ 0.001 ‘**’ 0.01 ‘*’ 0.05 ‘.’ 0.1 ‘ ’ 1
Approximate significance of smooth terms:
edf Ref.df F p-value
s(Petal.Width) 1 1 19.09 2.29e-05 ***
---
Signif. codes: 0 ‘***’ 0.001 ‘**’ 0.01 ‘*’ 0.05 ‘.’ 0.1 ‘ ’ 1
R-sq.(adj) = -2.64
lmer.REML = 57.675 Scale est. = 0.07373 n = 150
```

It shows a “significant” effect (of about .03) for the fixed effect and a “significant” smooth link with low complexity (as shown by the edf column).

When I run it with rstanarm:

```
fit <- stan_gamm4(Sepal.Width ~ Sepal.Length + s(Petal.Width), random=~(1|Species), data=iris, chains=1)
print(fit)
```

```
stan_gamm4
family: gaussian [identity]
formula: Sepal.Width ~ Sepal.Length + s(Petal.Width)
observations: 150
------
Median MAD_SD
(Intercept) 1.4 0.5
Sepal.Length 0.3 0.1
s(Petal.Width).1 0.0 0.1
s(Petal.Width).2 0.0 0.1
s(Petal.Width).3 0.0 0.1
s(Petal.Width).4 0.0 0.1
s(Petal.Width).5 0.0 0.1
s(Petal.Width).6 0.0 0.1
s(Petal.Width).7 -0.1 0.1
s(Petal.Width).8 0.0 0.1
s(Petal.Width).9 0.6 0.6
sigma 0.3 0.0
Smoothing terms:
Median MAD_SD
smooth_sd[s(Petal.Width)1] 0.2 0.1
smooth_sd[s(Petal.Width)2] 0.5 0.4
Error terms:
Groups Name Std.Dev.
Species (Intercept) 0.76
Residual 0.28
Num. levels: Species 3
Sample avg. posterior predictive distribution of y:
Median MAD_SD
mean_PPD 3.1 0.0
------
For info on the priors used see help('prior_summary.stanreg').
```

I retrieve in the output the coef fo sepal length (the “fixed” effect) but then, instead of having one smooth term, I have these 9 lines and I’m not sure what they refer to. I haven’t found the info in the docs.

Does anyone have an idea? Thanks a lot!