Create imputed datasets

Usage

impute(
  draws,
  references = NULL,
  update_strategy = NULL,
  strategies = getStrategies()
)

# S3 method for class 'random'
impute(
  draws,
  references = NULL,
  update_strategy = NULL,
  strategies = getStrategies()
)

# S3 method for class 'condmean'
impute(
  draws,
  references = NULL,
  update_strategy = NULL,
  strategies = getStrategies()
)

Arguments

draws

A draws object created by draws().

references

A named vector. Identifies the references to be used for reference-based imputation methods. Should be of the form c("Group1" = "Reference1", "Group2" = "Reference2"). If NULL (default), the references are assumed to be of the form c("Group1" = "Group1", "Group2" = "Group2"). This argument cannot be NULL if an imputation strategy (as defined by data_ice[[vars$strategy]] in the call to draws) other than MAR is set.

update_strategy

An optional data.frame. Updates the imputation method that was originally set via the data_ice option in draws(). See the details section for more information.

strategies

A named list of functions. Defines the imputation functions to be used. The names of the list should mirror the values specified in strategy column of data_ice. Default = getStrategies(). See getStrategies() for more details.

Details

impute() uses the imputation model parameter estimates, as generated by draws(), to first calculate the marginal (multivariate normal) distribution of a subject's longitudinal outcome variable depending on their covariate values. For subjects with intercurrent events (ICEs) handled using non-MAR methods, this marginal distribution is then updated depending on the time of the first visit affected by the ICE, the chosen imputation strategy and the chosen reference group as described in Carpenter, Roger, and Kenward (2013) . The subject's imputation distribution used for imputing missing values is then defined as their marginal distribution conditional on their observed outcome values. One dataset is being generated per set of parameter estimates provided by draws().

The exact manner in how missing values are imputed from this conditional imputation distribution depends on the method object that was provided to draws(), in particular:

• Bayes & Approximate Bayes: each imputed dataset contains 1 row per subject & visit from the original dataset with missing values imputed by taking a single random sample from the conditional imputation distribution.

• Conditional Mean: each imputed dataset contains 1 row per subject & visit from the bootstrapped or jackknife dataset that was used to generate the corresponding parameter estimates in draws(). Missing values are imputed by using the mean of the conditional imputation distribution. Please note that the first imputed dataset refers to the conditional mean imputation on the original dataset whereas all subsequent imputed datasets refer to conditional mean imputations for bootstrap or jackknife samples, respectively, of the original data.

• Bootstrapped Maximum Likelihood MI (BMLMI): it performs D random imputations of each bootstrapped dataset that was used to generate the corresponding parameter estimates in draws(). A total number of B*D imputed datasets is provided, where B is the number of bootstrapped datasets. Missing values are imputed by taking a random sample from the conditional imputation distribution.

The update_strategy argument can be used to update the imputation strategy that was originally set via the data_ice option in draws(). This avoids having to re-run the draws() function when changing the imputation strategy in certain circumstances (as detailed below). The data.frame provided to update_strategy argument must contain two columns, one for the subject ID and another for the imputation strategy, whose names are the same as those defined in the vars argument as specified in the call to draws(). Please note that this argument only allows you to update the imputation strategy and not other arguments such as the time of the first visit affected by the ICE. A key limitation of this functionality is that one can only switch between a MAR and a non-MAR strategy (or vice versa) for subjects without observed post-ICE data. The reason for this is that such a change would affect whether the post-ICE data is included in the base imputation model or not (as explained in the help to draws()). As an example, if a subject had their ICE on "Visit 2" but had observed/known values for "Visit 3" then the function will throw an error if one tries to switch the strategy from MAR to a non-MAR strategy. In contrast, switching from a non-MAR to a MAR strategy, whilst valid, will raise a warning as not all usable data will have been utilised in the imputation model.

References

James R Carpenter, James H Roger, and Michael G Kenward. Analysis of longitudinal trials with protocol deviation: a framework for relevant, accessible assumptions, and inference via multiple imputation. Journal of Biopharmaceutical Statistics, 23(6):1352–1371, 2013. [Section 4.2 and 4.3]

Examples

if (FALSE) { # \dontrun{

impute(
    draws = drawobj,
    references = c("Trt" = "Placebo", "Placebo" = "Placebo")
)

new_strategy <- data.frame(
  subjid = c("Pt1", "Pt2"),
  strategy = c("MAR", "JR")
)

impute(
    draws = drawobj,
    references = c("Trt" = "Placebo", "Placebo" = "Placebo"),
    update_strategy = new_strategy
)
} # }