1.5 NA | Basic Statistics

1.5 `NA`

NA is a logical constant of length 1 that contains a missing value indicator. The NA coding can identify some situations.

‘Not Available’, e.g. asking ‘what time is it?’ and there is no clock available.
‘Not Applicable’, e.g. asking ‘are you pregnant, sir?’
‘Not Announced/Answered’, e.g. being silent in an interrogation.
‘Not Accessed’, e.g. the student’s Moodle page was not accessed throughout the semester.

?NA
(x <- c(NA, 1:3, NA, 1000))

## [1]   NA    1    2    3   NA 1000

is.na(x)

## [1]  TRUE FALSE FALSE FALSE  TRUE FALSE

sum(is.na(x))

## [1] 2

anyNA(x)

## [1] TRUE

The VIM library by (Kowarik and Templ 2016) provides tools for viewing missing and imputed values.

library(VIM)
(a <- aggr(sleep, sortVars = TRUE))

## 
##  Variables sorted by number of missings: 
##  Variable      Count
##      NonD 0.22580645
##     Dream 0.19354839
##     Sleep 0.06451613
##      Span 0.06451613
##      Gest 0.06451613
##   BodyWgt 0.00000000
##  BrainWgt 0.00000000
##      Pred 0.00000000
##       Exp 0.00000000
##    Danger 0.00000000

## 
##  Missings in variables:
##  Variable Count
##      NonD    14
##     Dream    12
##     Sleep     4
##      Span     4
##      Gest     4

summary(a)

## 
##  Missings per variable: 
##  Variable Count
##   BodyWgt     0
##  BrainWgt     0
##      NonD    14
##     Dream    12
##     Sleep     4
##      Span     4
##      Gest     4
##      Pred     0
##       Exp     0
##    Danger     0
## 
##  Missings in combinations of variables: 
##         Combinations Count   Percent
##  0:0:0:0:0:0:0:0:0:0    42 67.741935
##  0:0:0:0:0:0:1:0:0:0     3  4.838710
##  0:0:0:0:0:1:0:0:0:0     2  3.225806
##  0:0:0:0:0:1:1:0:0:0     1  1.612903
##  0:0:1:0:1:0:0:0:0:0     2  3.225806
##  0:0:1:1:0:0:0:0:0:0     9 14.516129
##  0:0:1:1:0:1:0:0:0:0     1  1.612903
##  0:0:1:1:1:0:0:0:0:0     2  3.225806

References

Kowarik, Alexander, and Matthias Templ. 2016. “Imputation with the R Package VIM.” Journal of Statistical Software 74 (7): 1–16. https://doi.org/10.18637/jss.v074.i07.