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Storing and Analyzing Imputed Data with rbmiUtils

2025-05-20

1 Introduction

This vignette demonstrates how to:

Perform multiple imputation using the {rbmi} package.
Store and modify the imputed data using {rbmiUtils}.
Analyze the imputed data using:
- A standard ANCOVA on a continuous endpoint (CHG)
- A binary responder analysis on CRIT1FLN using {beeca}

This pattern enables reproducible workflows where imputation and analysis can be separated and revisited independently.

2 Statistical Context

This approach applies Rubin’s Rules for inference after multiple imputation:

We fit a model to each imputed dataset, dervive a response variable on the CHG score, extract marginal effects or other statistics of interest, and combine the results into a single inference using Rubin’s combining rules.

3 Step 1: Setup and Data Preparation

library(dplyr)
library(tidyr)
library(readr)
library(purrr)
library(rbmi)
library(beeca)
library(rbmiUtils)

set.seed(1974)

data("ADEFF")

ADEFF <- ADEFF %>%
  mutate(
    TRT = factor(TRT01P, levels = c("Placebo", "Drug A")),
    USUBJID = factor(USUBJID),
    AVISIT = factor(AVISIT)
  )

4 Step 2: Define Imputation Model

vars <- set_vars(
  subjid = "USUBJID",
  visit = "AVISIT",
  group = "TRT",
  outcome = "CHG",
  covariates = c("BASE", "STRATA", "REGION")
)

method <- method_bayes(
  n_samples = 100,
  control = control_bayes(warmup = 200, thin = 2)
)

dat <- ADEFF %>%
  select(USUBJID, STRATA, REGION, REGIONC, TRT, BASE, CHG, AVISIT)

draws_obj <- draws(data = dat, vars = vars, method = method)
#> 
#> SAMPLING FOR MODEL 'rbmi_mmrm' NOW (CHAIN 1).
#> Chain 1: 
#> Chain 1: Gradient evaluation took 0.000259 seconds
#> Chain 1: 1000 transitions using 10 leapfrog steps per transition would take 2.59 seconds.
#> Chain 1: Adjust your expectations accordingly!
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#> Chain 1:

impute_obj <- impute(draws_obj, references = c("Placebo" = "Placebo", "Drug A" = "Placebo"))

ADMI <- get_imputed_data(impute_obj)

5 Step 3: Add Responder Variables

ADMI <- ADMI %>%
  mutate(
    CRIT1FLN = ifelse(CHG > 3, 1, 0),
    CRIT1FL = ifelse(CRIT1FLN == 1, "Y", "N"),
    CRIT = "CHG > 3"
  )

6 Step 4: Continuous Endpoint Analysis (CHG)

ana_obj_ancova <- analyse_mi_data(
  data = ADMI,
  vars = vars,
  method = method,
  fun = ancova
)

pool_obj_ancova <- pool(ana_obj_ancova)
print(pool_obj_ancova)
#> 
#> Pool Object
#> -----------
#> Number of Results Combined: 100
#> Method: rubin
#> Confidence Level: 0.95
#> Alternative: two.sided
#> 
#> Results:
#> 
#>   ========================================================
#>       parameter      est     se     lci     uci     pval  
#>   --------------------------------------------------------
#>      trt_Week 24    -2.177  0.182  -2.535  -1.819  <0.001 
#>    lsm_ref_Week 24  0.077   0.131  -0.18   0.334   0.558  
#>    lsm_alt_Week 24   -2.1   0.125  -2.347  -1.854  <0.001 
#>      trt_Week 48    -3.806  0.256  -4.309  -3.304  <0.001 
#>    lsm_ref_Week 48  0.044   0.185  -0.319  0.408   0.811  
#>    lsm_alt_Week 48  -3.762  0.176  -4.107  -3.417  <0.001 
#>   --------------------------------------------------------

tidy_pool_obj(pool_obj_ancova)
#> # A tibble: 6 × 10
#>   parameter       description visit parameter_type lsm_type     est    se    lci
#>   <chr>           <chr>       <chr> <chr>          <chr>      <dbl> <dbl>  <dbl>
#> 1 trt_Week 24     Treatment … Week… trt            <NA>     -2.18   0.182 -2.53 
#> 2 lsm_ref_Week 24 Least Squa… Week… lsm            ref       0.0766 0.131 -0.180
#> 3 lsm_alt_Week 24 Least Squa… Week… lsm            alt      -2.10   0.125 -2.35 
#> 4 trt_Week 48     Treatment … Week… trt            <NA>     -3.81   0.256 -4.31 
#> 5 lsm_ref_Week 48 Least Squa… Week… lsm            ref       0.0442 0.185 -0.319
#> 6 lsm_alt_Week 48 Least Squa… Week… lsm            alt      -3.76   0.176 -4.11 
#> # ℹ 2 more variables: uci <dbl>, pval <dbl>

7 Step 5: Responder Endpoint Analysis (CRIT1FLN)

7.1 Define Analysis Function

gcomp_responder <- function(data, ...) {
  model <- glm(CRIT1FLN ~ TRT + BASE + STRATA + REGION, data = data, family = binomial)

  marginal_fit <- get_marginal_effect(
    model,
    trt = "TRT",
    method = "Ge",
    type = "HC0",
    contrast = "diff",
    reference = "Placebo"
  )

  res <- marginal_fit$marginal_results
  list(
    trt = list(
      est = res[res$STAT == "diff", "STATVAL"][[1]],
      se = res[res$STAT == "diff_se", "STATVAL"][[1]],
      df = NA
    )
  )
}

7.2 Define Variables and Run Analysis

vars_binary <- set_vars(
  subjid = "USUBJID",
  visit = "AVISIT",
  group = "TRT",
  outcome = "CRIT1FLN",
  covariates = c("BASE", "STRATA", "REGION")
)

ana_obj_prop <- analyse_mi_data(
  data = ADMI,
  vars = vars_binary,
  method = method,
  fun = gcomp_responder
)

pool_obj_prop <- pool(ana_obj_prop)
print(pool_obj_prop)
#> 
#> Pool Object
#> -----------
#> Number of Results Combined: 100
#> Method: rubin
#> Confidence Level: 0.95
#> Alternative: two.sided
#> 
#> Results:
#> 
#>   ==================================================
#>    parameter   est     se     lci     uci     pval  
#>   --------------------------------------------------
#>       trt     -0.063  0.012  -0.087  -0.039  <0.001 
#>   --------------------------------------------------

8 Final Notes

The ADMI object can be saved for later reuse.
Analyses can be modularly applied using custom functions.
The tidy output from tidy_pool_obj() is helpful for reporting and review.

These binaries (installable software) and packages are in development.
They may not be fully stable and should be used with caution. We make no claims about them.