# Bugs for pp_check, brms -to- emmeans communication, and strange "features"

• Operating System: Win7 * brms Version: 2.9.6 from GitHub

Following on @paul.buerkner advice Convergence fails for (every) truncated gaussian model I triggered a bunch of bugs. I still hope to add a “solution” to my original topic, so I started this separate topic.

First, lets see how pp_check() breaks down:

``````require("modelr")
require("tidyverse")
require("tidybayes")
require("brms")
require("emmeans")
set.seed(1)
nRats <- 20
rats <- data.frame(ratNo=1:nRats, mu=rnorm(nRats)/3, sigma=rlnorm(nRats),
TrtEffect=rnorm(nRats,mean=1)) # each rat is special
trueParams <- expand.grid(replicateNo=1:6, ratNo=1:nRats, Time=c(0,30,60)) %>%
mutate(Trt = ifelse(ratNo <= nRats/2, "Sham", "TBI"),
mu=rats\$mu[ratNo]+ifelse(Trt=="Sham", 0, rats\$TrtEffect[ratNo]*Time/10),
sigma = rats\$sigma[ratNo] + 1 + mu/3)
data.measured <- trueParams %>%
mutate( pa = rnorm(n(), mean=mu, sd=sigma), # the "real" effect
mpa = abs(round(pa*2)/2) )  # and its rounded measurement, producing many zeroes
data.recorded <- data.measured %>% ungroup %>%
mutate(Trt = as.factor(Trt), Time = as.factor(Time), ratNo = as.factor(ratNo) ) %>%
group_by(Trt, Time, ratNo) %>% summarize(mpa = mean(mpa))

# Paul's advise that mpa==0 break truncated normal model
data.recorded2 <- data.recorded %>% mutate(mpa = replace(mpa, mpa < 0.25/6, 0.25/6))

tn.prior <- brm(
data = data.recorded2, family = gaussian,
mpa | trunc(lb=0) ~ Trt*Time,
prior = c( prior(exponential(10), class = sigma), # comment sigma prior the get model with best loo
prior(normal(0, 3), class = "Intercept"),
prior(normal(0, 3), class = "b") ),
seed = 1, chains=3, cores=3, iter=1e4, sample_prior = "only" )
pp_check(tn.prior)
# Using 10 posterior samples for ppc type 'dens_overlay' by default.
# Error: NAs not allowed in 'yrep'.
# Call `rlang::last_error()` to see a backtrace
``````

I tried to understand @paul.buerkner advice from Help understanding and setting informative priors in brms that priors should be checked with pp_check(), but it broke down (all Rhat==1.0 and each ESS >= 8512 for tn.prior model).

I usually fiddle better with priors, when I progressively tighten them (in Statistical Rethinking 2 style https://github.com/rmcelreath/statrethinking_winter2019) to the point when 95% CI from marginal_effects() contain all data points,while not being 10^BigNumber - times wider than the actual data. The code below shows either a bug or a strange feature: textual summary me[[1]] reports upper__ bound as Inf, but graphical summary, if you plot it, will draw all upper margins to be slightly above 20…

``````me <- marginal_effects(tn.prior, "Trt:Time", method="fitted")
me[[1]]
#    Trt Time      mpa cond__ effect1__ effect2__ estimate__     se__    lower__ upper__
# 1 Sham    0 3.294444      1      Sham         0   3.048649 3.818217 0.01706015     Inf
# 2 Sham   30 3.294444      1      Sham        30   3.327312 4.188858 0.01816852     Inf
# ....

plot(me, points = TRUE, point_args = list(width = 0.1), plot=FALSE)[[1]] +
geom_hline(yintercept=0, linetype="dashed", color="black")
``````

Next are the two inconsistent features of update(): (a) the progress report not redirected to RStudio’s Viewer and (b) some arguments of the model, like “chains” or “iter”, will be inherited by updater, while cores parameter will be quietly set to 1.

``````tn.fit <-  update(tn.prior, sample_prior="yes", seed = 1)
# The desired updates require recompiling the model
# Compiling the C++ model
# Start sampling
#
# SAMPLING FOR MODEL '42ba15d28a59f71a914bbab12cbba47d' NOW (CHAIN 1).
# Chain 1: Rejecting initial value:
# Chain 1:   Log probability evaluates to log(0), i.e. negative infinity.
# Chain 1:   Stan can't start sampling from this initial value.
# Chain 1:
# Chain 1: Gradient evaluation took 0 seconds
# Chain 1: 1000 transitions using 10 leapfrog steps per transition would take 0 seconds.
# Chain 1: Adjust your expectations accordingly!
# Chain 1:
# Chain 1:
# Chain 1: Iteration:    1 / 10000 [  0%]  (Warmup)
# Chain 1: Iteration: 1000 / 10000 [ 10%]  (Warmup)

summary(tn.fit) # model converged ...
pp_check(tn.fit) # ... and now pp_check works without error
``````

And now I report the most annoying bug: for some distributions brms is not passing the transformation to “response scale” correctly into emmeans:

``````(emm <- emmeans(tn.fit, ~ Trt | Time, transform = "response"))
# ....
# Time = 60:
# Trt  emmean lower.HPD upper.HPD
# Sham -0.902     -4.03     1.856
# TBI   7.163      5.28     8.991
#
# HPD interval probability: 0.95

data.recorded2 %>%
group_by(Trt, Time) %>%
data_grid(ratNo=NA) %>%
# add_fitted_draws(tn.fit, scale="response") %>%  # same as marginal_effects(tn.fit, "Trt:Time", method="fitted")
add_fitted_draws(tn.fit, scale="linear") %>%  # emmeans can ONLY do this
median_hdi()
#   Trt   Time  ratNo  .row .value .lower .upper .width .point .interval
# ...
# 3 Sham  60    NA        3 -0.902  -4.03  1.86    0.95 median hdi
# ...
# 6 TBI   60    NA        6  7.16    5.28  8.99    0.95 median hdi
``````

This bug also affects lognormal and hurdle_lognormal, and maybe some other distributions which I did not check. This is definitely not a feature, since some other distributions work well, for example bernoulli:

``````some.fake.bernoulli.data <- data.recorded %>% mutate(YesNo = factor(mpa>5,c(FALSE,TRUE)))
b.fit <- brm( data = some.fake.bernoulli.data, family = bernoulli,
YesNo ~ Trt*Time,
prior = c(prior(normal(0, 1), class = "Intercept"),
prior(normal(0, 1), class = "b")),
seed = 1, chains=3, cores=3)

(emm <- emmeans(b.fit, ~ Trt | Time))
# Time = 60:
#  Trt  emmean lower.HPD upper.HPD
#  Sham -2.417    -3.958    -0.906
#  TBI   0.353    -0.824     1.510
#
# Results are given on the logit (not the response) scale.
# HPD interval probability: 0.95
# Warning message:
#   In model.matrix.default(trms, m, contrasts.arg = contr) :
#   variable 'YesNo' is absent, its contrast will be ignored
``````

… emmeans by default report “linear scale” and could be commanded to use “response scale”:

``````(emm <- emmeans(b.fit, ~ Trt | Time, transform = "response"))
# Time = 60:
#  Trt  response lower.HPD upper.HPD
#  Sham   0.0818   0.00889     0.226
#  TBI    0.5874   0.33068     0.841
#
# HPD interval probability: 0.95
# Warning message:
#   In model.matrix.default(trms, m, contrasts.arg = contr) :
#   variable 'YesNo' is absent, its contrast will be ignored
``````

Sure, the warning message flashes unnecessary red text into user’s eyes, but at least it is not affecting the results as far as I can tell.

Sorry for spilling many bugs in one topic. I really-really hope fixing 'em will make our beloved brms better.

The first problem with the NAs is that your priors are so wide that their predictions cannot be evaluated properly at the lower bound which may lead to NAs eventually in the prediction of the truncated values. There is nothing (to my knowledge) that I can do from the brms side about this.

That upper is still plotted despite being Inf seems to be something that ggplot2 does for some reason. Again, nothing that I can do from the brms side.

I don’t know why the progress being not redirected to Rstudio but since rstan handles all the interaction with the viewer, I, again, cannot do anything from the brms side about it. Both in the usual call to brm and in update.brmsfit the same rstan code is called for the model fitting. Could it be that is is not redirected because you didn’t set cores to something > 1 in the call to update (see also below)? At least on my machine it works as expected with cores > 1.

Cores is not a parameter that affects the model but only how efficiently it is estimated. It is not stored anywhere, neither in the stanfit nor in the brmsfit object. Also, I don’t want to call anything in parallel without explicit instruction by the user. This may be annoying for you but I am afraid is intentional.

Lastly, the emmeans interface to brms is not written by me and lives in emmeans itself not in brms. It does not support a lot of brms models properly, yet. After brms 3.0 I may look at it myself and see if I can provide a consistent interface but for now I would only use emmeans with more standard brmsfit models.

I am sorry that I don’t have any better news. I am always happy to fix problems in brms and usually fix them whithin 2 days (as you see in the brms issue tracker), but for the problems you reported there is nothing on the brms side which I can do (or the behavior is intentional as explained above).

Dear Paul, thank you for looking into these issues. I must apologize for being a little fired up from one of my coworkers complaining to my boss about my “reproducible rmarkdown report” not reproducing after his “trivial changes”. I found a way to work around these issues, which (surprisingly) are all solvable by re-running the same R chunk again several times until it finally works. Now I handle the NA-related errors from `pp_check` by wrapping it in `tryCatch`. The “lost connections” and/or other “transient” model fitting issues are, of course, more serous: I call `brm` and save results to file myself, disable the `brm` code chunk, and make report proceed by loading the fitted model.

Thank you for promissing that emmeans-related issues will receive your attention after brms 3.0. I will “wait and hope”, as emmeans is so very convenient when I need to maintain several reports, relying on pretty similar models.

With regard to saving the brmsfit object, you can also simply use the `file` argument of `brm` for this purpose.