brm (version 1.1.1)

brm: Fitting Binary Regression Models

Description

brm is used to estimate the association between two binary variables, and how that varies as a function of other covariates.

Usage

brm(
  y,
  x,
  va,
  vb = NULL,
  param,
  est.method = "MLE",
  vc = NULL,
  optimal = TRUE,
  weights = NULL,
  subset = NULL,
  max.step = NULL,
  thres = 1e-08,
  alpha.start = NULL,
  beta.start = NULL,
  message = FALSE
)

Arguments

y

The response vector. Should only take values 0 or 1.

x

The exposure vector. Should only take values 0 or 1.

va

The covariates matrix for the target model. It can be specified via an object of class "formula" or a matrix. In the latter case, no intercept terms will be added to the covariates matrix.

vb

The covariates matrix for the nuisance model. It can be specified via an object of class "formula" or a matrix. In the latter case, no intercept terms will be added to the covariates matrix. (If not specified, defaults to va.)

param

The measure of association. Can take value 'RD' (risk difference), 'RR' (relative risk) or 'OR' (odds ratio)

est.method

The method to be used in fitting the model. Can be 'MLE' (maximum likelihood estimation, the default) or 'DR' (doubly robust estimation).

vc

The covariates matrix for the probability of exposure, often called the propensity score. It can be specified via an object of class "formula" or a matrix. In the latter case, no intercept terms will be added to the covariates matrix. By default we fit a logistic regression model for the propensity score. (If not specified, defaults to va.)

optimal

Use the optimal weighting function for the doubly robust estimator? Ignored if the estimation method is 'MLE'. The default is TRUE.

weights

An optional vector of 'prior weights' to be used in the fitting process. Should be NULL or a numeric vector.

subset

An optional vector specifying a subset of observations to be used in the fitting process.

max.step

The maximal number of iterations to be passed into the optim function. The default is 1000.

thres

Threshold for judging convergence. The default is 1e-6.

alpha.start

Starting values for the parameters in the target model.

beta.start

Starting values for the parameters in the nuisance model.

message

Show optimization details? Ignored if the estimation method is 'MLE'. The default is FALSE.

Value

A list consisting of

param

the measure of association.

point.est

the point estimates.

se.est

the standard error estimates.

cov

estimate of the covariance matrix for the estimates.

conf.lower

the lower limit of the 95% (marginal) confidence interval.

conf.upper

the upper limit of the 95% (marginal) confidence interval.

p.value

the two sided p-value for testing zero coefficients.

coefficients

the matrix summarizing key information: point estimate, 95% confidence interval and p-value.

param.est

the fitted RR/RD/OR.

va

the matrix of covariates for the target model.

vb

the matrix of covariates for the nuisance model.

converged

Did the maximization process converge?

Details

brm contains two parts: the target model for the dependence measure (RR, RD or OR) and the nuisance model; the latter is required for maximum likelihood estimation. If param="RR" then the target model is \(log RR(va) = \alpha*va\). If param="RD" then the target model is \(atanh RD(va) = \alpha*va\). If param="OR" then the target model is \(log OR(va) = \alpha*va\). For RR and RD, the nuisance model is for the log Odds Product: \(log OP(vb) = \beta*vb\). For OR, the nuisance model is for the baseline risk: \(logit(P(y=1|x=0,vb)) = \beta*vb.\) In each case the nuisance model is variation independent of the target model, which ensures that the predicted probabilities lie in \([0,1]\). See Richardson et al. (2016+) for more details.

If est.method="DR" then given a correctly specified logistic regression model for the propensity score \(logit(P(x=1|vc)) = \gamma*vc\), estimation of the RR or RD is consistent, even if the log Odds Product model is misspecified. This estimation method is not available for the OR. See Tchetgen Tchetgen et al. (2014) for more details.

When estimating RR and RD, est.method="DR" is recommended unless it is known that the log Odds Product model is correctly specified. Optimal weights (optimal=TRUE) are also recommended to increase efficiency.

For the doubly robust estimation method, MLE is used to obtain preliminary estimates of \(\alpha\), \(\beta\) and \(\gamma\). The estimate of \(\alpha\) is then updated by solving a doubly-robust estimating equation. (The estimate for \(\beta\) remains the MLE.)

References

Thomas S. Richardson, James M. Robins and Linbo Wang. "On Modeling and Estimation for the Relative Risk and Risk Difference." Journal of the American Statistical Association: Theory and Methods (2017).

Eric J. Tchetgen Tchetgen, James M. Robins and Andrea Rotnitzky. "On doubly robust estimation in a semiparametric odds ratio model." Biometrika 97.1 (2010): 171-180.

See Also

getProbScalarRD, getProbRD (vectorised), getProbScalarRR and getProbRR (vectorised) for functions calculating risks P(y=1|x=1) and P(y=1|x=0) from (atanh RD, log OP) or (log RR, log OP);

predict.blm for obtaining fitted probabilities from brm fits.

Examples

# NOT RUN {
set.seed(0)
n           = 100
alpha.true  = c(0,-1)
beta.true   = c(-0.5,1)
gamma.true  = c(0.1,-0.5)
params.true = list(alpha.true=alpha.true, beta.true=beta.true, 
     gamma.true=gamma.true)
v.1         = rep(1,n)       # intercept term
v.2         = runif(n,-2,2) 
v           = cbind(v.1,v.2)
pscore.true = exp(v %*% gamma.true) / (1+exp(v %*% gamma.true))
p0p1.true   = getProbRR(v %*% alpha.true,v %*% beta.true)
x           = rbinom(n, 1, pscore.true)  
pA.true       = p0p1.true[,1]
pA.true[x==1] = p0p1.true[x==1,2]
y = rbinom(n, 1, pA.true)

fit.mle = brm(y,x,v,v,'RR','MLE',v,TRUE)
fit.drw = brm(y,x,v,v,'RR','DR',v,TRUE)
fit.dru = brm(y,x,v,v,'RR','DR',v,FALSE)

fit.mle2 = brm(y,x,~v.2, ~v.2, 'RR','MLE', ~v.2,TRUE) # same as fit.mle

# }