Hello, I am having trouble generating posterior predictions using posterior_survfit(). I am trying to use a new data frame, but it is not using the new data frame and instead is using values from the dataset I used to fit the model. The fitted variables in the model are New.Treatment (6 treatments = categorical), Openness (a continuous light index min= 2.22, mean= 6.903221 and max=10.54), subplot_by_site(categorical-720 sites), New.Species.name(categorical- 165 species). My new data frame has 94 rows and the posterior_survfit() is giving me 3017800 rows. Help, please!

```
head(nd)
New.Treatment Openness
1 BE 5
2 BE 6
3 BE 7
4 BE 8
5 BE 9
6 BE 10
fit= stan_surv(formula = Surv(days, Status_surv) ~ New.Treatment*Openness + (1 |subplot_by_site)+(1|New.Species.name),
data = dataset,
basehaz = "weibull",
chains=4,
iter = 2000,
cores =4 )
Post=posterior_survfit(fit, type="surv",
newdata=nd5)
head(Post)
id cond_time time median ci_lb ci_ub
1 1 NA 62.0000 0.9626 0.9623 1.0000
2 1 NA 69.1313 0.9603 0.9600 0.9997
3 1 NA 76.2626 0.9581 0.9579 0.9696
4 1 NA 83.3939 0.9561 0.9557 0.9665
5 1 NA 90.5253 0.9541 0.9537 0.9545
6 1 NA 97.6566 0.9522 0.9517 0.9526
#here some sample data set to replicate my problem
library(rstanarm)
data_NHN<- expand.grid(New.Treatment = c("A","B","C"), Openness = c(seq(2, 11, by=0.15)))
data_NHN$subplot_by_site=c(rep("P1",63),rep("P2",60),rep("P3",60))
data_NHN$Status_surv=sample(0:1,183, replace=TRUE)
data_NHN$New.Species.name=c(rep("sp1",10),rep("sp2",40),rep("sp1",80),rep("sp2",20),rep("sp1",33))
data_NHN$days=sample(10, size = nrow(data_NHN), replace = TRUE)
nd_t<- expand.grid(New.Treatment = c("A","B","C"), Openness = c(seq(2, 11, by=1)))
mod= stan_surv(formula = Surv(days, Status_surv) ~ New.Treatment+Openness + (1 |subplot_by_site)+(1|New.Species.name),
data =data_NHN,
basehaz = "weibull",
chains=4,
iter = 30,
cores =4)
summary(mod)
pos=posterior_survfit(mod, type="surv",
newdataEvent=nd_t,
times = 0)
head(pos)
# I am interested in predicting values for specific Openess values (nd_t=20 rows)but I am getting instead values for each point in time (pos=18300rows) .
```

- Operating System: Mac OS Catalina 10.15.6
- R version: 4.0
- rstan version: 2.21.2
- rstanarm Version: rstanarm_2.21.2

Any suggestions on why is it not working. itâ€™s not clear how to give some sort of plot of the effects of one variable in the interaction as the other changes and the associated uncertainty (i.e. a marginal effects plot). TIA.