brm_data {brms.mmrm}

Create and preprocess an MMRM dataset.

Description

Create a dataset to analyze with an MMRM.

Usage

brm_data(
  data,
  outcome,
  baseline = NULL,
  group,
  subgroup = NULL,
  time,
  patient,
  covariates = character(0L),
  missing = NULL,
  reference_group,
  reference_subgroup = NULL,
  reference_time = NULL,
  role = NULL,
  level_baseline = NULL,
  level_control = NULL
)

Arguments

data

Data frame or tibble with longitudinal data.

outcome

Character of length 1, name of the continuous outcome variable. Example possibilities from clinical trial datasets include "CHG" and "AVAL". The outcome column in the data should be a numeric vector.

baseline

Character of length 1, name of the baseline response variable (for example, "BASE" in many clinical trial datasets). Only relevant if the response variable is change from baseline. Supply NULL to ignore or omit.

group

Character of length 1, name of the treatment group variable. Example possibilities from clinical trial datasets include "TRT01P", "TREATMENT", "TRT", and "GROUP". The group column in the data should be a character vector or unordered factor.

subgroup

Character of length 1, optional name of the a discrete subgroup variable. Set to NULL to omit the subgroup (default). If present, the subgroup column in the data should be a character vector or unordered factor.

time

Character of length 1, name of the discrete time variable. Example possibilities from clinical trial datasets include "AVISIT" and "VISIT". For most analyses, please ensure the time column in the data is an ordered factor. You can easily turn the time variable into an ordered factor using brm_data_chronologize(), either before or immediately after brm_data() (but before any ⁠brm_archetype_*()⁠ functions). This ensures the time points sort in chronological order, which ensures the correctness of informative prior archetypes and autoregressive / moving average correlation structures.

Ordinarily, ordered factors automatically use polynomial contrasts from contr.poly(). This is undesirable for MMRMs, so if the time variable is an ordered factor, then brm_data() manually sets contrasts(data[[time]]) to a set of treatment contrasts using contr.treatment(). If you prefer different contrasts, please manually set contrasts(data[[time]]) to something else after calling brm_data().

patient

Character of length 1, name of the patient ID variable. Example possibilities from clinical trial datasets include "USUBJID", "SUBJID", "PATIENT", "PATIENTID", "SUBJECT", "SUBJIDID", "SBJID", "STYSID1A", "SBJ1N", and "ID". The patient column in the data should be a factor or character vector.

covariates

Character vector of names of other covariates. All these covariates are assumed to be non-time-varying. For time-varying covariates, please manually expand the data to the full grid of patients and time points before you call brm_data(). See the "Preprocessing" section for details.

missing

Character of length 1, name of an optional variable in a simulated dataset to indicate which outcome values should be missing. Set to NULL to omit.

reference_group

Atomic value of length 1, Level of the group column to indicate the control group. Example possibilities from clinical trial datasets include "Placebo", "PLACEBO", "PBO", "PLB", "CONTROL", "CTRL", "REFERENCE", and "REF". reference_group only applies to the post-processing that happens in functions like brm_marginal_draws() downstream of the model. It does not control the fixed effect mapping in the model matrix that brms derives from the formula from brm_formula().

reference_subgroup

Atomic value of length 1, level of the subgroup column to use as a reference for pairwise differences in when computing marginal means downstream of the model. It does not control the fixed effect mapping in the model matrix that brms derives from the formula from brm_formula().

reference_time

Atomic value of length 1 or NULL, level of the time column to indicate the baseline time point. Leave as NULL if there is no baseline or baseline is not included in data[[time]].

If reference_time is not NULL, then brm_marginal_draws() will calculate change from baseline, and it will calculate treatment differences as differences between change-from-baseline values. If reference_time is not NULL, then brm_marginal_draws() will not calculate change from baseline, and it will calculate treatment differences as differences between response values.

Note: reference_time only applies to the post-processing that happens in functions like brm_marginal_draws() downstream of the model. It does not control the fixed effect mapping in the model matrix that brms derives from the formula from brm_formula().

role

Deprecated as unnecessary on 2024-07-11 (version 1.0.1.9007). Use reference_time to supply a baseline time point value if it exists.

level_baseline

Deprecated on 2024-01-11 (version 0.2.0.9002). Use reference_time instead.

level_control

Deprecated on 2024-01-11 (version 0.2.0.9002). Use reference_group instead.

Value

A classed tibble with attributes which denote features of the data such as the treatment group and discrete time variables.

Preprocessing

The preprocessing steps in brm_data() are as follows:

• Perform basic assertions to make sure the data and other arguments are properly formatted.

• Convert the group and time columns to character vectors.

• Sanitize the levels of the group and time columns using make.names(unique = FALSE, allow_ = TRUE) to ensure agreement between the data and the output of brms.

• For each implicitly missing outcome observation, add explicit row with the outcome variable equal to NA_real_. Missing values in the predictors are implicitly filled using zoo::na.locf() on within each patient, which is not valid for time-varying covariates. If any covariates are time-varying, please manually perform this step before calling brm_data().

• Arrange the rows of the data by group, then patient, then discrete time.

• Select only the columns of the data relevant to an MMRM analysis.

Separation string

Post-processing in brm_marginal_draws() names each of the group-by-time marginal means with the delimiting character string from Sys.getenv("BRM_SEP", unset = "|"). Neither the column names nor element names of the group and time variables can contain this string. To set a custom string yourself, use Sys.setenv(BRM_SEP = "YOUR_CUSTOM_STRING").

See Also

Other data: brm_data_change(), brm_data_chronologize()

Examples

set.seed(0)
data <- brm_simulate_simple()$data
colnames(data) <- paste0("col_", colnames(data))
data
brm_data(
  data = data,
  outcome = "col_response",
  group = "col_group",
  time = "col_time",
  patient = "col_patient",
  reference_group = "group_1",
  reference_time = "time_1"
)