The semideviation is a risk measurement tool similar to the standard deviation but with superior estimation accuracy. The semideviation function computes the sample lower and upper semideviation around the mean for multiple variables. It serve to separate the downside risk from the upside risk.
In measuring risk, the standard deviation will calculate the overall dispersion around a mean or expected value with disregard for the proportion of the downside and upside dispersions. On the other hand, the semideviation quantifies the magnitude of the downside and upside deviations solely.