measure of dispersion
standard deviation
Orlaith Burke
Alejandra Gonzalez-Beltran
Philippe Rocca-Serra
The standard deviation of a random variable, statistical population, data set, or probability distribution is a measure of variation which correspond to the average distance from the mean of the data set to any given point of that dataset. It also corresponds to the square root of its variance.
http://en.wikipedia.org/wiki/Standard_deviation
sd(x, na.rm = FALSE)
http://stat.ethz.ch/R-manual/R-patched/library/stats/html/sd.html
σ
ready for release