qgcomp (version 2.7.0)

qgcomp.zi.noboot: Quantile g-computation for zero-inflated count outcomes under linearity/additivity

Description

This function estimates a linear dose-response parameter representing a one quantile increase in a set of exposures of interest for zero-inflated count outcomes. This function is limited to linear and additive effects of individual components of the exposure. This model estimates the parameters of a marginal structural zero-inflated model (MSM) based on g-computation with quantized exposures. Note: this function is valid only under linear and additive effects of individual components of the exposure, but when these hold the model can be fit with very little computational burden.

Usage

qgcomp.zi.noboot(
  f,
  data,
  expnms = NULL,
  q = 4,
  breaks = NULL,
  id = NULL,
  weights,
  alpha = 0.05,
  bayes = FALSE,
  ...
)

Arguments

f

R style formula using syntax from 'pscl' package: depvar ~ indvars_count | indvars_zero

data

data frame

expnms

character vector of exposures of interest

q

NULL or number of quantiles used to create quantile indicator variables representing the exposure variables. If NULL, then gcomp proceeds with un-transformed version of exposures in the input datasets (useful if data are already transformed, or for performing standard g-computation)

breaks

(optional) NULL, or a list of (equal length) numeric vectors that characterize the minimum value of each category for which to break up the variables named in expnms. This is an alternative to using 'q' to define cutpoints.

id

(optional) NULL, or variable name indexing individual units of observation (only needed if analyzing data with multiple observations per id/cluster)

weights

"case weights" - passed to the "weight" argument of zeroinfl.

alpha

alpha level for confidence limit calculation

bayes

not yet implemented

...

arguments to zeroinfl (e.g. dist)

Value

a qgcompfit object, which contains information about the effect measure of interest (psi) and associated variance (var.psi), as well as information on the model fit (fit) and information on the weights/standardized coefficients in the positive (pos.weights) and negative (neg.weights) directions.

Details

A zero-inflated version of quantile g-computation based on the implementation in the 'pscl' package. A zero-inflated distribution is a mixture distribution in which one of the distributions is a point mass at zero (with probability given by a logistic model), and the other distribution is a discrete or continuous distribution. This estimates the effect of a joint increase in all exposures on 1) the odds of belonging to the "zero" vs. "count" portions of the distribution and/or 2) the rate parameter for the "count" portion of the distribution.

See Also

qgcomp.zi.boot,qgcomp.noboot, qgcomp.cox.noboot, and zeroinfl

Examples

# NOT RUN {
set.seed(50)
n=100
dat <- data.frame(y=rbinom(n, 1, 0.5)*rpois(n, 1.2), x1=runif(n), x2=runif(n), z=runif(n))

# poisson count model, mixture in both portions
qgcomp.zi.noboot(f=y ~ z + x1 + x2 | x1 + x2, expnms = c('x1', 'x2'), 
    data=dat, q=2, dist="poisson")
    
# negative binomial count model, mixture and covariate in both portions
qgcomp.zi.noboot(f=y ~ z + x1 + x2 | z + x1 + x2, expnms = c('x1', 'x2'), 
   data=dat, q=2, dist="negbin")  
qgcomp.zi.noboot(f=y ~ z + x1 + x2, expnms = c('x1', 'x2'), 
   data=dat, q=2, dist="negbin") # equivalent
   
# negative binomial count model, mixture only in the 'count' portion of the model
qgcomp.zi.noboot(f=y ~ z + x1 + x2 | z, expnms = c('x1', 'x2'), data=dat, q=2, dist="negbin")

# weighted analysis
dat$w = runif(n)*5
qgcomp.zi.noboot(f=y ~ z + x1 + x2 | x1 + x2, expnms = c('x1', 'x2'), 
    data=dat, q=2, dist="poisson", weights=w)
# Expect this:     
# Warning message:
# In eval(family$initialize) : non-integer #successes in a binomial glm!

# }