Hi Simsco, thanks for the great question! This is exactly the sort of thing DeclareDesign was built to answer.

So, luckily the folk in charge of `broom.mixed`

have made some tidy methods for `lmer`

objects – anything that works with `tidy()`

is fairly easy to integrate into the DD workflow.

Let’s load the packages and build a multilevel DGP with some assumptions baked in:

```
library(DeclareDesign)
library(tidyverse)
library(lme4)
library(broom.mixed)
N_schools <- 1
N_classes <- 10
N_students_per_class <- 20
N_tests <- 3
ml_design <-
# Tests are clustered within students, students within classes,
# classes within schools. Each level has a level specific effect, e.g. each i
# student in class j gets shock alpha_j ~ N(0,.001), etc.
declare_population(
school = add_level(N = N_schools, school_effect = rnorm(N, 0, .001)),
class = add_level(N = N_classes, class_effect = rnorm(N, 0, .001),
N_students = rpois(N,N_students_per_class) + 5),
student = add_level(N = N_students,
test_ability = runif(N, -.01, .01),
student_baseline = runif(N, min = .4,max = .8)),
test = add_level(N = 3, test_effect = rnorm(N, test_ability, .001))) +
# Each student's score on a given test is a function of their baseline,
# plus the school, class and test-specific effects -- AND the treatment effect
# is heterogeneous by all three (random slopes and intercepts). Note here that
# the mean of the k'th test effect is specific to the i'th student:
# test_ability_i ~ Uniform(-.01, .01); test_effect_ik ~ N(test_ability_i, .001)
declare_potential_outcomes(
score ~ student_baseline + (1 + treatment) * (school_effect + class_effect + test_effect),
assignment_variable = "treatment") +
# We want the average treatment effect across tests (but we could reparameterize this
# as the average effect on the student's average score)
declare_estimand(ate = mean(score_treatment_1 - score_treatment_0)) +
# I'm assuming that the treatment is clustered at the class-level (easy to change)
declare_assignment(prob = .5, clusters = class, assignment_variable = "treatment") +
# Reveal the score potential outcome as a function of the treatment
declare_reveal(score, treatment)
```

You can look at the data using `draw_data(ml_design) %>% head()`

.

Now, we build a little helper function for the `lmer()`

call, to ensure it returns estimates as a data.frame:

```
# We will use the fact that broom.mixed has a tidy method for lmer:
tidy_lmer <- function(data, formula){
lmer_fit <- lmer(formula = formula, data = data)
tidy_fit <- broom.mixed::tidy(lmer_fit)
subset(tidy_fit, term == "treatment")
}
```

And we add it to the design. Notice that we are also using `dplyr`

in the second-to-last step to aggregate test scores to the student-level average, before running our regression estimator.

```
ml_design <-
ml_design +
# lmer
declare_estimator(formula = score ~ treatment + (1 | student),
handler = tidy_estimator(tidy_lmer),
estimand = "ate",
label = "Random Intercept") +
# Aggregate the data to student level using dplyr
declare_step(handler = function(data){
data %>% group_by(school,class,student,treatment) %>%
summarize(av_score = mean(score))},
label = "Aggregate to student level") +
# Run a regression of average scores on treatment
declare_estimator(formula = av_score ~ treatment,
model = lm_robust,
clusters = class,
estimand = "ate",
label = "Average Score")
```

Unfortunately, it seems as though `lmer()`

does not automatically include p-values and confidence intervals, so you’re not going to get power and coverage from the diagnosis. If you need these, you could use the standard errors in the `tidy_fit`

object that is created inside `tidy_lmer`

to add a column of p-values and confidence intervals to the output. Then DD will also calculate power and coverage. But this will at least get you bias, RMSE, and things like the average SE versus the standard deviation of the estimates across simulations.

You’ll want to run a large number of simulations (e.g. 1000), but here’s what it looks like when you run 5:

```
simulate_design(ml_design,sims = 5)
diagnose_design(ml_design,sims = 5)
```

Cheers