smcfcs (version 1.7.1)

smcfcs.casecohort: Substantive model compatible fully conditional specification imputation of covariates for case cohort studies

Description

Multiply imputes missing covariate values using substantive model compatible fully conditional specification for case cohort studies.

Usage

smcfcs.casecohort(
  originaldata,
  smformula,
  sampfrac,
  in.subco,
  method,
  predictorMatrix = NULL,
  m = 5,
  numit = 10,
  rjlimit = 1000,
  noisy = FALSE,
  errorProneMatrix = NULL
)

Arguments

originaldata

The case-cohort data set (NOT a full cohort data set with a case-cohort substudy within it)

smformula

A formula of the form "Surv(entertime,t,d)~x", where d is the event (d=1) or censoring (d=0) indicator, t is the event or censoring time and entertime is equal to the time origin (typically 0) for individuals in the subcohort and is equal to (t-0.001) for cases outside the subcohort [this sets cases outside the subcohort to enter follow-up just before their event time. The value 0.001 may need to be modified depending on the time scale.]

sampfrac

The proportion of individuals from the underlying full cohort who are in the subcohort

in.subco

The name of a column in the dataset with 0/1s that indicates whether the subject is in the subcohort

method

A required vector of strings specifying for each variable either that it does not need to be imputed (""), the type of regression model to be be used to impute. Possible values are "norm" (normal linear regression), "logreg" (logistic regression), "brlogreg" (bias reduced logistic regression), "poisson" (Poisson regression), "podds" (proportional odds regression for ordered categorical variables), "mlogit" (multinomial logistic regression for unordered categorical variables), or a custom expression which defines a passively imputed variable, e.g. "x^2" or "x1*x2". "latnorm" indicates the variable is a latent normal variable which is measured with error. If this is specified for a variable, the "errorProneMatrix" argument should also be used.

predictorMatrix

An optional predictor matrix. If specified, the matrix defines which covariates will be used as predictors in the imputation models (the outcome must not be included). The i'th row of the matrix should consist of 0s and 1s, with a 1 in the j'th column indicating the j'th variable be used as a covariate when imputing the i'th variable. If not specified, when imputing a given variable, the imputation model covariates are the other covariates of the substantive model which are partially observed (but which are not passively imputed) and any fully observed covariates (if present) in the substantive model. Note that the outcome variable is implicitly conditioned on by the rejection sampling scheme used by smcfcs, and should not be specified as a predictor in the predictor matrix.

m

The number of imputed datasets to generate. The default is 5.

numit

The number of iterations to run when generating each imputation. In a (limited) range of simulations good performance was obtained with the default of 10 iterations. However, particularly when the proportion of missingness is large, more iterations may be required for convergence to stationarity.

rjlimit

Specifies the maximum number of attempts which should be made when using rejection sampling to draw from imputation models. If the limit is reached when running a warning will be issued. In this case it is probably advisable to increase the rjlimit until the warning does not appear.

noisy

logical value (default FALSE) indicating whether output should be noisy, which can be useful for debugging or checking that models being used are as desired.

errorProneMatrix

An optional matrix which if specified indicates that some variables are measured with classical measurement error. If the i'th variable is measured with error by variables j and k, then the (i,j) and (i,k) entries of this matrix should be 1, with the remainder of entries 0. The i'th element of the method argument should then be specified as "latnorm". See the measurement error vignette for more details.

Author

Ruth Keogh

Jonathan Bartlett

Details

This version of smcfcs is designed for use with case cohort studies but where the analyst does not wish to, or cannot (due to not having the necessary data) impute the full cohort. The function's arguments are the same as for the main smcfcs function, except for smformula, in.subco, and sampfrac - see above for details on how these should be specified.

Examples

#the following example is not run when the package is compiled on CRAN
#(to keep computation time down), but it can be run by package users
if (FALSE) {
  #as per the documentation for ex_cc, the sampling fraction is 10%
  imps <- smcfcs.casecohort(ex_cc, smformula="Surv(entertime, t, d)~x+z", sampfrac=0.1,
                            in.subco="in.subco", method=c("", "", "norm", "", "", "", ""))
  library(mitools)
  impobj <- imputationList(imps$impDatasets)
  models <- with(impobj, coxph(Surv(entertime,t,d)~x+z+cluster(id)))
  summary(MIcombine(models))
}