It is well-known that for value investors, risk is not standard deviation stemming from prices. Rather, it is the product of the expected loss times the probability of that loss occuring. One of the best efforts in defining risk that I have encountered comes from Seth Klarman (Trades, Portfolio), who, in one of his investor letters, provided a detailed explanation on how he thinks and acts upon this definition of risk.
“The risk of an investment is described by both the probability and the potential amount of loss. The risk of an investment—the probability of an adverse outcome—is partly inherent in its very nature. A dollar spent on biotechnology research is a riskier investment than a dollar used to purchase utility equipment. The former has both a greater probability of loss and a greater percentage of the investment at stake.
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