Total Risk, Diversifiable Risk and Nondiversifiable Risk: A Pedagogic Note
重新定义了证券风险中可分散与不可分散部分，要求不可分散风险完全决定均衡市场价格，尤其适用于负贝塔证券。
The decomposition of a security risk into diversifiable (or unsystematic) and nondiversifiable (or systematic) risks has emerged from the portfolio approach of capital investment and has culminated in the well-known Capital Asset Pricing Model (CAPM), developed by Sharpe [4], Lintner [3] and others. In this framework, the diversifiable risk is the risk that can be “washed out” by diversification and the nondiversifiable risk is the risk which cannot be diversified away. It appears to us that the decomposition of risk into its components is in some cases vague and in most cases imprecise. We define the diversifiable and nondiversifiable risk measures as two complementary components of the standard deviation of a security's rate of return. Furthermore, we require thatthe nondiversifiable risk measure will completely determine its equilibrium market price. We shall see that the definition presented is appealing for all securities and particularly for those with negative Beta. To be more specific, recall that a security's β is given by the slope of the following time series regression: