DEFINITION of 'Downside Deviation'
A measure of downside risk that focuses on returns that fall below a minimum threshold or minimum acceptable return (MAR). It is used in the calculation of a risk measure known as the Sortino Ratio.
INVESTOPEDIA EXPLAINS 'Downside Deviation'
Standard deviation, the most widely used measure of investment risk, has some limitations, such as the fact that it treats all deviations from the average  whether positive or negative  as the same. However, investors are generally more concerned with negative divergences than positive ones, i.e. downside risk is a bigger concern. Downside deviation resolves this issue by focusing only on downside risk.
Another advantage over standard deviation is that downside deviation can also be tailored to the specific objectives and risk profile of different investors who have various levels of minimum acceptable return.

Sharpe Ratio
A ratio developed by Nobel laureate William F. Sharpe to measure ... 
Sortino Ratio
A modification of the Sharpe ratio that differentiates harmful ... 
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Standard Deviation
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