SimDesign (version 1.1)

add_missing: Add missing values to a vector given a MCAR, MAR, or MNAR scheme

Description

Given an input vector, replace elements of this vector with missing values according to some scheme. Default method replaces input values with a MCAR scheme (where on average 10% of the values will be replaced with NAs). MAR and MNAR are supported by replacing the default FUN argument.

Usage

add_missing(y, fun = function(y, rate = 0.1, ...) rep(rate, length(y)), ...)

Arguments

y

an input vector that should contain missing data in the form of NA's

fun

a user defined function indicating the missing data mechanism for each element in y. Function must return a vector of probability values with the length equal to the length of y. Each value in the returned vector indicates the probability that the respective element in y will be replaced with NA. Function must contain the argument y, representing the input vector, however any number of additional arguments can be included

...

additional arguments to be passed to FUN

Value

the input vector y with the sampled NA values (according to the FUN scheme)

Details

Given an input vector y, and other relevant variables inside (X) and outside (Z) the data-set, the three types of missingness are:

Examples

set.seed(1)
y <- rnorm(1000)

## 10% missing rate with default FUN
head(ymiss <- add_missing(y), 10)

## 50% missing with default FUN
head(ymiss <- add_missing(y, rate = .5), 10)

## missing values only when female and low
X <- data.frame(group = sample(c('male', 'female'), 1000, replace=TRUE),
                level = sample(c('high', 'low'), 1000, replace=TRUE))
head(X)

fun <- function(y, X, ...){
    p <- rep(0, length(y))
    p[X$group == 'female' & X$level == 'low'] <- .2
    p
}

ymiss <- add_missing(y, X, fun=fun)
tail(cbind(ymiss, X), 10)

## missingness as a function of elements in X (i.e., a type of MAR)
fun <- function(y, X){
   # missingness with a logistic regression approach
   df <- data.frame(y, X)
   mm <- model.matrix(y ~ group + level, df)
   cfs <- c(-5, 2, 3) #intercept, group, and level coefs
   z <- cfs %*% t(mm)
   plogis(z)
}

ymiss <- add_missing(y, X, fun=fun)
tail(cbind(ymiss, X), 10)

## missing values when y elements are large (i.e., a type of MNAR)
fun <- function(y) ifelse(abs(y) > 1, .4, 0)
ymiss <- add_missing(y, fun=fun)
tail(cbind(y, ymiss), 10)