Downside risk is a type of financial risk that investors must take into consideration when investing in securities. It is the potential of a security’s value to decrease when market conditions deteriorate, resulting in a decrease in the security’s price. Generally, the higher the downside risk of a security the greater the amount of capital an investor needs to invest in order to make a profitable return.

When calculating downside risk, there are a number of approaches to consider. One of the most popular methods is semi-deviation. This method of downside risk analysis looks at the expected return and the downside risk of a security. It subtracts the projected return from the worst losses that could occur in the security, resulting in an estimation of the downside risk. Another approach is VaR, or value at risk, which looks at the probability of an investor experiencing a loss over a specified period of time. It takes into account the current market conditions and looks at the possible volatility of the security within that time frame, giving a potential risk of loss. Lastly, a Roy's Safety First ratio takes into account the expected return, risk of loss, and the possibility of improvement from the current market conditions. This calculation can help an investor determines if a security should be kept in an investment portfolio.

Overall, investors must be mindful of the potential for downside risk when investing in securities. Different security and investments carry different levels of downside risk, and so investors must be aware of this when deciding what to invest in. While downside analysis can help protect investors against losses, they should also consider the upside potential of their investment. The key is to understand the risks involved and develop an appropriate strategy to manage risk exposure.