Paradox or At Least Variance Found: A Comment on “Mean-Variance Approaches to Risk-Return Relationships in Strategy: Paradox Lost”
指出，在缺乏额外信息或假设的情况下，计算出的均值-方差关系无法与随时间变化的关系效应区分开，因此无法在实证系统内确认其性质或推广到其他时间段。
In general, the problem is that the computed mean-variance relationship for a period of time cannot be identified in distinction to the effects of shifts in the relationship over time—without additional information or assumptions. Thus, using a mean-variance approach to risk-return relationships means that statements about the nature of the mean-variance association cannot be confirmed in a nontrivial fashion within the empirical system nor generalized to any other time period—including subperiods. (Ruefli 1990) (emphasis in original)