Estimating Betas and Stock-Return Correlations From Monthly Data: A Warning Note.

The empirical finance literature makes extensive use of 'monthly' stock returns, where a monthly return is the change in stock price between one particular day of the calendar month - which we term the reference day - and the corresponding day of the following month. We show that estimates of betas and stock-market correlations are highly sensitive to the choice of reference day and we suggest that studies based on such estimates can be unreliable. We support this claim by carrying out two small-scale empirical studies showing in each case that the results of critical tests are dependent upon the choice of reference day.betas, international correlations, estimation risk

Implied Standard Deviations and Post-earnings Announcement Volatility

This paper investigates volatility increases following annual earnings announcements. Standard deviations implied by options prices are used to show that announcements of bad news result in a lower volatility increase than those of good news, and delay the increase by a day. Reports that are difficult to interpret also delay the volatility increase. This delay is incremental to that caused by reporting bad news, although the effect of bad news on slowing down the reaction time is dominant. It is argued that the delays reflect market uncertainty about the implications of the news. Copyright Blackwell Publishers Ltd 2002.