dualSD: Dual Standard Deviations

In Hmisc: Harrell Miscellaneous

Dual Standard Deviations

Description

Computes one standard deviation for the lower half of the distribution of a numeric vector and another SD for the upper half. By default the center of the distribution for purposes of splitting into "halves" is the mean. The user may override this with center. When splitting into halves, observations equal to the center value are included in both subsets.

Usage

dualSD(x, na.rm = FALSE, nmin = 10, center = xbar)

Arguments

x	a numeric vector
na.rm	set to TRUE to find any NA values and remove them before computing SDs.
nmin	the minimum number of non-NA obesrvations that must be present for two SDs to be computed. If the mumber of non-missing values falls below nmin, the regular SD is duplicated in the result.
center	center point for making the two subsets. The sample mean is used to compute the two SDs no matter what is specified for center.

Details

The purpose of dual SDs is to describe variability for asymmetric distributions. Symmetric distributions are also handled, though slightly less efficiently than a single SD does.

Value

a 2-vector of SDs with names bottom and top

Author(s)

Frank Harrell

See Also

pMedian()

Examples

set.seed(1)
x <- rnorm(20000)
sd(x)
dualSD(x)
y <- exp(x)
s1 <- sd(y)
s2 <- dualSD(y)
s1
s2
quantile(y, c(0.025, 0.975))
mean(y) + 1.96 * c(-1, 1) * s1
mean(y) + 1.96 * c(- s2['bottom'], s2['top'])
c(mean=mean(y), pseudomedian=pMedian(y), median=median(y))

Hmisc documentation built on Nov. 6, 2025, 5:07 p.m.