robincar_glm: Covariate adjustment using generalized linear working model

In RobinCar: Robust Inference for Covariate Adjustment in Randomized Clinical Trials

Covariate adjustment using generalized linear working model

Description

Estimate treatment-group-specific response means and (optionally) treatment group contrasts using a generalized linear working model.

Usage

robincar_glm(
  df,
  treat_col,
  response_col,
  formula = NULL,
  car_strata_cols = NULL,
  car_scheme = "simple",
  g_family = stats::gaussian,
  g_accuracy = 7,
  contrast_h = NULL,
  contrast_dh = NULL,
  variance_type = 1
)

Arguments

df	A data.frame with the required columns
treat_col	Name of column in df with treatment variable
response_col	Name of the column in df with response variable
formula	The formula to use for adjustment specified using as.formula("..."). This overrides car_strata_cols and covariate_cols.
car_strata_cols	Names of columns in df with car_strata variables
car_scheme	Name of the type of covariate-adaptive randomization scheme. One of: "simple", "pocock-simon", "biased-coin", "permuted-block".
g_family	Family that would be supplied to glm(...), e.g., binomial. If no link specified, will use default link, like behavior in glm. If you wish to use a negative binomial working model with an unknown dispersion parameter, then use 'g_family="nb"'.
g_accuracy	Level of accuracy to check prediction un-biasedness.
contrast_h	An optional function to specify a desired contrast
contrast_dh	An optional jacobian function for the contrast (otherwise use numerical derivative)
variance_type	The type of variance estimator to use, type 1, 2, or 3. All three are asymptotically equivalent. See details for more.

Details

The output is the AIPW estimator given by (for each treatment group a):

\frac{1}{n} \sum_{i=1}^{n} \hat{\mu}_a(X_i) + \frac{1}{n_a} \sum_{i:A_i=a} \{Y_i - \hat{\mu}(X_i)\}

where Y_i is the outcome, A_i is the treatment assignment, X_i are the covariates, n_a = \sum_{i=1}^{n} A_i=a, and \hat{\mu}_a is the estimated conditional mean function based on the GLM working model. This working model has treatment a-specific coefficients if 'adj_method' is "heterogeneous". Otherwise, they are shared across the treatment arms. Alternatively, if 'formula' is used, the working model can be specified according to the user.

Importantly, the estimated variance accounts for misspecification of the working model, and for covariate-adaptive randomization. The variance estimator is given by

\hat{V} = \hat{V}_{\rm SR} - \hat{V}_{\Omega}

where \hat{V}_{\rm SR} is the contribution to the variance under simple randomization, and \hat{V}_{\Omega} is a term that only appears when a covariate-adaptive randomization scheme is used. The \hat{V}_{\Omega} is the second line of \hat{V} in \insertCitebannickGeneralFormCovariate2025RobinCar.

There are three different estimators available for \hat{V}_{\rm SR}, which the user can choose with the argument variance_type. We describe these here.

The three variance types are given as follows:

• Type 1 (default):

  \mathrm{diag}\left[\hat{\pi}_a^{-1} (\mathrm{Var}_a(Y_i) - 2\hat{Q}_{a,a} + \hat{\Sigma}_{a,a} ), a = 1, \dots, K \right] + \hat{Q} + \hat{Q}^T - \hat{\Sigma}

• Type 2:

  \mathrm{diag}\left[\hat{\pi}_a^{-1} (\mathrm{Var}_a(Y_i - \hat{\mu}_a(X_i)) - 2\hat{Q}_{a,a} + \hat{\Sigma}_{a,a} ), a = 1, \dots, K \right] + \hat{Q} + \hat{Q}^T - \hat{\Sigma}

• Type 3:

  \mathrm{diag}\left[\hat{\pi}_a^{-1} E_a([Y_i - \hat{\mu}_a(X_i)]^2), a = 1, \dots, K \right] + \hat{A}

where \hat{\pi}_a is the treatment proportion for group a, \hat{Q}_{a,b} = \mathrm{Cov}_a(Y_i, \hat{\mu}_b(X_i)), \hat{\Sigma}_{a,b} = \mathrm{Cov}(\hat{\mu}_a(X_i), \hat{\mu}_b(X_i)), and the matrix \hat{A} has diagonal entries for (a, a) given by

2\mathrm{Cov}_a(Y_i - \hat{\mu}_a(X_i), \hat{\mu}_a(X_i)) + \mathrm{Var}_a(\hat{\mu}_a(X_i))

and off-diagonal entries for (a, b) given by

\mathrm{Cov}_a(Y_i, \hat{\mu}_b(X_i)) + \mathrm{Cov}_b(Y_i, \hat{\mu}_a(X_i)) - (1/2) \left[\mathrm{Cov}_a(\hat{\mu}_a(X_i), \hat{\mu}_b(X_i)) + \mathrm{Cov}_b(\hat{\mu}_a(X_i), \hat{\mu}_b(X_i)) \right].

We use E_a, \mathrm{Var}_a, and \mathrm{Cov}_a to refer to the empirical expectation, variance, and covariance among observations in group a only, and \mathrm{Cov} is the covariance within the entire sample.

Please see the Supplemental Material Sect. H of \insertCitebannickGeneralFormCovariate2025RobinCar for a discussion of the merits of each type of variance estimator. Briefly, we recommend variance types 1 generally, and variance type 3 if it is anticipated that the distribution of X varies substantially over treatment groups.

Value

If 'contrast_h' argument is used, outputs a 'main' and a 'contrast' object. The 'main' object has the following structure:

result	A dplyr::tibble() with the treatment label, treatment mean estimate using AIPW, estimated SE, and p-value based on a z-test with estimate and SE.
varcov	The variance-covariance matrix for the treatment mean estimates.
settings	List of model settings used in covariate adjustment.
original_df	The original dataset provided by the user.
mod	The fit from the glm() working model used for covariate adjustment.
mu_a	Predicted potential outcomes for each treatment category (columns) and individual (rows). These are the \hat{\mu}_a

.

g.estimate	The G-computation estimate based only on \frac{1}{n} \sum_{i=1}^{n} \hat{\mu}_a(X_i). This is equivalent to the AIPW estimate when a canonical link function is used.
data	Attributes about the dataset.

The 'contrast' object has a structure that is documented in RobinCar::robincar_contrast().

References

\insertAllCited

RobinCar documentation built on June 8, 2025, 12:12 p.m.