[R] sd(): why error with NA's?

[Raubertas, Richard]

R 1.8.1 with Windows XP.

I have a question about how sd() behaves with NA's:

> mean(c(1,2,3,NA))
[1] NA
> median(c(1,2,3,NA))
[1] NA
> mad(c(1,2,3,NA))
[1] NA
> sd(c(1,2,3,NA))
Error in var(x, na.rm = na.rm) : missing observations in cov/cor
>

What is so special about the standard deviation, relative to
other descriptive statistics, that the presence of NA's 
deserves an error instead of simply returning NA?
(I know about na.rm=TRUE, but that is not the point here.)

A few small changes to sd() would seem to resolve the anomaly:

sd <- function(x, na.rm=FALSE)
# Function like built-in 'sd', but return NA instead of error when
# 'na.rm' is FALSE and 'x' has NA's.
{
    if (is.matrix(x)) {
        apply(x, 2, sd, na.rm = na.rm)
    } else if (is.vector(x)) {
        if (!na.rm && any(is.na(x)))  NA
        else sqrt(var(x, na.rm = na.rm))
    } else if (is.data.frame(x)) {
        sapply(x, sd, na.rm = na.rm)
    } else {
        x <- as.vector(x)
        if (!na.rm && any(is.na(x)))  NA
        else sqrt(var(x, na.rm = na.rm))
    }
}

Rich Raubertas
Merck & Co.


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