ABSTRACT Models of asset pricing generally assume that the variables which characterize the state of the economy are observable. However, the distributional properties of asset prices that are relevant for portfolio decisions are in general not observable, and therefore must be estimated. The estimation of expected returns is a particularly difficult problem and estimation errors are likely to be substantial. In this light, it is reasonable to examine whether the assumption of observability of expected returns and other relevant state variables causes significant mis‐specification in equilibrium models of asset prices. This paper has three main objectives: first, to derive optimal estimators for the unobservable expected instantaneous returns using observations of past realized returns; second, to establish that estimation and portfolio choice can be solved in two separate steps; third, to analyze the impact of estimation error on investment choices. The estimators of expected returns are in general not consistent, i.e., the estimation error does not tend to disappear asymptotically. The effects of the estimation error, therefore, cannot be ignored even if realized returns are observed continuously over an infinite time period.

[An important element of a firm's disclosure strategy is the timing of its mandatory public announcements. In this article, two aspects of disclosure timing are examined. The first is the intraday timing of earnings announcements. It is demonstrated here that, under reasonable conditions, market prices reflect better the valuation implications of an earnings announcement when it is made during trading hours rather than after the market has closed. This implies that managers should prefer to release earnings with positive (negative) implications for firm value during (after) trading hours. The second issue examined is the sequencing of multiple corporate disclosures. It is shown that if the announcements have positive (negative) implications for firm value, managers should prefer to make them separately (simultaneously), as market prices better reflect the valuation implications of multiple announcements when they are made at different times.]

In the absence of significant news, hedging strategies were blamed for the stock market crash of October 1987; but traditional models cannot explain how a relatively small amount of selling could cause so large a price drop. We develop a rational expectations model in which prices play an important role in shaping expectations; markets are much less liquid in our model than in traditional models. Discontinuities (or "crashes") can occur even with relatively little hedging. The model is consistent with theories as disparate as Keynes' "beauty contest" insight and Thom's "catastrophe" analysis and suggests means to reduce volatility.

An important element of a firm's disclosure strategy is the timing of its mandatory public announcements. In this article, two aspects of disclosure timing are examined. The first is the intraday timing of earnings announcements. It is demonstrated here that, under reasonable conditions, market prices reflect better the valuation implications of an earnings announcement when it is made during trading hours rather than after the market has closed. This implies that managers should prefer to release earnings with positive (negative) implications for firm value during (after) trading hours. The second issue examined is the sequencing of multiple corporate disclosures. It is shown that if the announcements have positive (negative) implications for firm value, managers should prefer to make them separately (simultaneously), as market prices better reflect the valuation implications of multiple announcements when they are made at different times.

In the absence of significant news, hedging strategies were blamed for the stock market crash of October 1987; but traditional models cannot explain how a relatively small amount of selling could cause so large a price drop. The authors develop a rational expectations model in which prices play an important role in shaping expectations; markets are much less liquid in their model than in traditional models. Discontinuities (or "crashes") can occur even with relatively little hedging. The model is consistent with theories as disparate as Keynes' "beauty contest" insights and Thom's "catastrophe" analysis and suggests means to reduce volatility. Copyright 1990 by American Economic Association.