浅析力学中动量守恒与角动量守恒

在力学中动量守恒和角动量守恒是学生容易出错的问题。本文分析了子弹击入木棒问题与子弹击入沙袋问题的区别,指出什么情况下系统的角动量守恒,什么情况下系统的动量守恒,从而说明了理论上的实际应用。     

Analysts’ Momentum Recommendations

We document that the likelihood of analyst recommendations following past stock returns decreased abruptly in 2003, coinciding with the Global Settlement and other regulatory changes designed to restrain analysts’ conflicts of interest. We also document that the likelihood of recommendations following past stock returns is abnormally high for recommendations issued after negative stock returns (but not for those issued after positive stock returns), among inexperienced and inaccurate analysts, among large brokerage houses, and for companies with high share turnover. Moreover, the recommendations that are more likely to follow past stock returns are accompanied by earnings forecast revisions that are larger in magnitude and less accurate ex post. Overall, our findings suggest that analysts with conflicts of interest and limited ability are more likely to base their recommendations on past stock returns. Finally, we document that the recommendations that are more likely to follow past stock returns (especially those that were issued before 2003 and those that are issued after negative stock returns) contribute to existing price momentum by generating incrementally stronger short‐term and long‐term stock returns.     

Introduction

This study decomposes the unconditional stock return volatility into two categories: systematic versus idiosyncratic risk, to re-examine the link between size and risk in the banking industry. The feasibility of the model is tested using data for US banks from 1998 to 2007. The evidence uncovered suggests that the practice of size-related diversification obtained with large banks reduces the firm-specific risk, and thus weakens stock return variances. However, rather than eliminating firm-specific risk, it is being transformed into systematic risk. Additionally, our empirical findings can potentially explain why a bank's size-related diversification does not result in a reduction in its unconditional stock return volatility reported in Demsetz and Straha [Historical patterns and recent changes in the relationship between bank size and risk. Federal Reserve Bank of New York Economic Policy Review, 1(2), 13–26 (1995); Diversification, size, and risk at bank holding companies. Journal of Money, Credit, and Banking, 29, 300–313 (1997)].     

The Moving Average Ratio and Momentum

I show the ratio of the short‐term moving average to the long‐term moving average (moving average ratio, MAR) has significant predictive power for future returns. The MAR combined with nearness to the 52‐week high explains most of the intermediate‐term momentum profits. This suggests that an anchoring bias, in which investors use moving averages or the 52‐week high as reference points for estimating fundamental values, is the primary source of momentum effects. Momentum caused by the anchoring bias do not disappear in the long‐run even when there are return reversals, confirming that intermediate‐term momentum and long‐term reversals are separate phenomena.     

Macroeconomic interdependence in East Asia

This paper analyzes macroeconomic interdependence among 10 Asian economies. In this connection, we decompose their macroeconomic activities (real GDP) into common and country-specific components using the Bai–Ng method (2004). Our results suggest first that both components are non-stationary and have permanent effects on their overall economy. Second, we find the relative importance of common factors in all countries in terms of their contribution to variations in real GDP. But evidence is also obtained for country-specific effects becoming increasingly important in countries like China in recent years. Therefore, if, for example, China is expected to grow at a fast pace in future, our findings imply that creation of a regional monetary union of these 10 countries needs to be held back until the Chinese economy has become more dominant in the region.     

An Evaluation of Testing Procedures for Long Horizon Event Studies

We conduct a comprehensive simulation study to evaluate testing procedures for long horizon event studies. The simulation results raise the following concerns about some popular practices: (1) using the four-factor model that includes the Fama-French three factors and a momentum-related factor causes serious over rejection of the null hypothesis; (2) using reference portfolios as benchmark tends to overestimate event firms' long-term returns; and (3) the computation-intensive bootstrap test has low power for long event horizons. Moreover, unless the number of event firms in a study is very large, all testing procedures suffer substantial loss of power quickly as event horizon increases, especially for samples of small firms. Of particular interest, the combination of the nonparametric sign test with a single firm benchmark shows the best performance consistently in our simulations.     

Institutional Herding in the ADR Market

This study examines institutional herding in the ADR market between 1985 and 1998. We find a significant positive relation between changes in institutional ownership and ADR returns over the same period. The positive relation persists after we control for the momentum effect in the US stock markets. We also find that in the ADR market, past winners (losers) in the herding period continue to be the winners (losers) in the post-herding period. The lack of a returns reversal suggests institutional herding is related to momentum trading. However, the positive relation between institutional ownership changes and ADR returns remains after controlling for momentum trading in the ADR market. Our results also rule out that positive feedback trading is related to institutional herding in the ADR market.     

证券投资基金惯性反转投资行为实证研究

文章依据行为金融的理论和研究方法,通过改进的GTM模型,以基金重仓持有的股票相对于上证综合指数的超常收益率来构造赢家组合和输家组合。对24支偏股型开放式基金和49支偏股型封闭式基金,从1999年1月到2006年3月的投资行为进行研究。结果表明在交易策略上,中国的证券投资基金整体采用惯性交易策略,但倾向于买过去表现好的股票,尤其是收益率高于同期上证综合指数收益率的股票;不倾向于卖出过去表现差的股票,即采用高买高卖的策略。惯性交易估计值在考虑有新股票进入时显著大于其他各种情况,这表明基金经理在选择新股进入投资组合时追涨的行为十分明显。     

物流矢量分析方法初探

分析物流的特点,定义物流动量、物流通量等概念,阐述了物流动量、物流通量概念的内涵。在此基础上运用矢量分析的方法,研究“均匀发货,匀速行驶”理想条件下物流活动的基本规律,得出了物流中心服务能力、客户需求量等要素数量关系的规律,为定量研究物流问题奠定理论基础。