A measure of the dispersion of all observations that fall below the mean or target value of a data set. Semivariance is an average of the squared deviations of values that are less than the mean. The formula for semivariance is as follows:
Where:
n = the total number of observations below the mean
r = the observed value
average = the mean or target value of the data set
Investopedia Says:
Semivariance is similar to variance; however, it only considers observations below the mean. A useful tool in portfolio or asset analysis, semivariance provides a measure for downside risk. While standard deviation and variance provide measures of volatility, semivariance only looks at the negative fluctuations of an asset. By neutralizing all values above the mean, or an investor's target return, semivariance estimates the average loss that a portfolio could incur.
For risk averse investors, solving for optimal portfolio allocations by minimizing semivariance would limit the likelihood of a large loss.
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a variogram, others use the terms variogram and semivariogram synonymously. However, Bachmaier and Backes (2008), who discussed this confusion, have shown that 
should be called a variogram, terms like semivariogram or semivariance should be avoided. This also shall become the guideline of the agricultural journal Precision Agriculture.
