How do I handle multiple kinds of missingness in R?

Many surveys have codes for different kinds of missingness. For instance, a codebook might indicate:

0-99 Data

-1 Question not asked

-5 Do not know

-7 Refused to respond

-9 Module not asked

Stata has a beautiful facility for handling these multiple kinds of missingness, in that it allows you to assign a generic . to missing data, but more specific kinds of missingness (.a, .b, .c, ..., .z) are allowed as well. All the commands which look at missingness report answers for all the missing entries however specified, but you can sort out the various kinds of mi开发者_Go百科ssingness later on as well. This is particularly helpful when you believe that refusal to respond has different implications for the imputation strategy than does question not asked.

I have never run across such a facility in R, but I would really like to have this capability. Are there any ways of marking several different types of NA? I could imagine creating more data (either a vector of length nrow(my.data.frame) containing the types of missingness, or a more compact index of which rows had what types of missingness), but that seems pretty unwieldy.

I know what you look for, and that is not implemented in R. I have no knowledge of a package where that is implemented, but it's not too difficult to code it yourself.

A workable way is to add a dataframe to the attributes, containing the codes. To prevent doubling the whole dataframe and save space, I'd add the indices in that dataframe instead of reconstructing a complete dataframe.

eg :

NACode <- function(x,code){
    Df <- sapply(x,function(i){
        i[i %in% code] <- NA
        i
    })

    id <- which(is.na(Df))
    rowid <- id %% nrow(x)
    colid <- id %/% nrow(x) + 1
    NAdf <- data.frame(
        id,rowid,colid,
        value = as.matrix(x)[id]
    )
    Df <- as.data.frame(Df)
    attr(Df,"NAcode") <- NAdf
    Df
}

This allows to do :

> Df <- data.frame(A = 1:10,B=c(1:5,-1,-2,-3,9,10) )
> code <- list("Missing"=-1,"Not Answered"=-2,"Don't know"=-3)
> DfwithNA <- NACode(Df,code)
> str(DfwithNA)
'data.frame':   10 obs. of  2 variables:
 $ A: num  1 2 3 4 5 6 7 8 9 10
 $ B: num  1 2 3 4 5 NA NA NA 9 10
 - attr(*, "NAcode")='data.frame':      3 obs. of  4 variables:
  ..$ id   : int  16 17 18
  ..$ rowid: int  6 7 8
  ..$ colid: num  2 2 2
  ..$ value: num  -1 -2 -3

The function can also be adjusted to add an extra attribute that gives you the label for the different values, see also this question. You could backtransform by :

ChangeNAToCode <- function(x,code){
    NAval <- attr(x,"NAcode")
    for(i in which(NAval$value %in% code))
        x[NAval$rowid[i],NAval$colid[i]] <- NAval$value[i]

    x
}

> Dfback <- ChangeNAToCode(DfwithNA,c(-2,-3))
> str(Dfback)
'data.frame':   10 obs. of  2 variables:
 $ A: num  1 2 3 4 5 6 7 8 9 10
 $ B: num  1 2 3 4 5 NA -2 -3 9 10
 - attr(*, "NAcode")='data.frame':      3 obs. of  4 variables:
  ..$ id   : int  16 17 18
  ..$ rowid: int  6 7 8
  ..$ colid: num  2 2 2
  ..$ value: num  -1 -2 -3

This allows to change only the codes you want, if that ever is necessary. The function can be adapted to return all codes when no argument is given. Similar functions can be constructed to extract data based on the code, I guess you can figure that one out yourself.

But in one line : using attributes and indices might be a nice way of doing it.

The most obvious way seems to use two vectors:

• Vector 1: a data vector, where all missing values are represented using NA. For example, c(2, 50, NA, NA)
• Vector 2: a vector of factors, indicating the type of data. For example, factor(c(1, 1, -1, -7)) where factor 1 indicates the a correctly answered question.

Having this structure would give you a create deal of flexibility, since all the standard na.rm arguments still work with your data vector, but you can use more complex concepts with the factor vector.

Update following questions from @gsk3

1. Data storage will dramatically increase: The data storage will double. However, if doubling the size causes real problem it may be worth thinking about other strategies.
2. Programs don't automatically deal with it. That's a strange comment. Some functions by default handle NAs in a sensible way. However, you want to treat the NAs differently so that implies that you will have to do something bespoke. If you want to just analyse data where the NA's are "Question not asked", then just use a data frame subset.
3. now you have to manipulate two vectors together every time you want to conceptually manipulate a variable I suppose I envisaged a data frame of the two vectors. I would subset the data frame based on the second vector.
4. There's no standard implementation, so my solution might differ from someone else's. True. However, if an off the shelf package doesn't meet your needs, then (almost) by definition you want to do something different.

I should state that I have never analysed survey data (although I have analysed large biological data sets). My answers above appear quite defensive, but that's not my intention. I think your question is a good one, and I'm interested in other responses.

This is more than just a "technical" issue. You should have a thorough statistical background in missing value analysis and imputation. One solution requires playing with R and ggobi. You can assign extremely negative values to several types of NA (put NAs into margin), and do some diagnostics "manually". You should bare in mind that there are three types of NA:

• MCAR - missing completely at random, where P(missing|observed,unobserved) = P(missing)
• MAR - missing at random, where P(missing|observed,unobserved) = P(missing|observed)
• MNAR - missing not at random (or non-ignorable), where P(missing|observed,unobserved) cannot be quantified in any way.

IMHO this question is more suitable for CrossValidated.

But here's a link from SO that you may find useful:

Handling missing/incomplete data in R--is there function to mask but not remove NAs?

You can dispense with NA entirely and just use the coded values. You can then also roll them up to a global missing value. I often prefer to code without NA since NA can cause problems in coding and I like to be able to control exactly what is going into the analysis. If have also used the string "NA" to represent NA which often makes things easier.

-Ralph Winters

I usually use them as values, as Ralph already suggested, since the type of missing value seems to be data, but on one or two occasions where I mainly wanted it for documentation I have used an attribute on the value, e.g.

> a <- NA
> attr(a, 'na.type') <- -1
> print(a)
[1] NA
attr(,"na.type")
[1] -1

That way my analysis is clean but I still keep the documentation. But as I said: usually I keep the values.

Allan.

I´d like to add to the "statistical background component" here. Statistical analysis with missing data is a very good read on this.