Tongshu Ma

Risk Reduction in Large Portfolios: Why Imposing the Wrong Constraints Helps

Mean-variance efficient portfolios constructed using sample moments often involve taking extreme long and short positions. Hence practitioners often impose portfolio weight constraints when constructing efficient portfolios. Green and Hollifield (1992) argue that the presence of a single dominant factor in the covariance matrix of returns is why we observe extreme positive and negative weights. If this were the case then imposing the weight constraint should hurt whereas the empirical evidence is often to the contrary. We reconcile this apparent contradiction. We show that constraining portfolio weights to be nonnegative is equivalent to using the sample covariance matrix after reducing its large elements and then form the optimal portfolio without any restrictions on portfolio weights. This shrinkage helps reduce the risk in estimated optimal portfolios even when they have negative weights in the population. Surprisingly, we also find that once the nonnegativity constraint is imposed, minimum variance portfolios constructed using the monthly sample covariance matrix perform as well as those constructed using covariance matrices estimated using factor models, shrinkage estimators, and daily data. When minimizing tracking error is the criterion, using daily data instead of monthly data helps. However, the sample covariance matrix without any correction for microstructure effects performs the best.

Tick Size, NYSE Rule 118, and Ex-Dividend Day Stock Price Behavior

Bali and Hite (1998) and Dubofsky (1992) propose models in which market microstructure effects play a role in the ex-dividend day price drop anomaly. Bali and Hite suggest that the anomaly is caused solely by price discreteness, while Dubofsky suggests that NYSE Rule 118 is also involved. We test these models by examining cum- to ex-day price drops during the one-eighth, one-sixteenth, and decimal tick size regimes. While the evidence is qualitatively consistent with Dubofskys predictions, neither model is satisfactory in a quantitative sense. One of our main empirical findings is that no significant decline was evident in the magnitude of the ex-day anomaly after the tick size reduction.

The 52-Week High Momentum Strategy in International Stock Markets

We study the 52-week high momentum strategy in international stock markets proposed by George and Hwang [George, T., Hwang, C.Y., 2004. The 52-week high and momentum investing. Journal of Finance 59, 2145-2176.]. This strategy produces profits in 18 of the 20 markets studied, and the profits are significant in 10 markets. The 52-week high momentum profits exist independently from the Jegadeesh and Titman [Jegadeesh, N., Titman, S., 1993. Returns to buying winners and selling losers: implications for market efficiency. Journal of Finance 48, 65-91.] individual stock and Moskowitz and Grinblatt [Moskowitz, T.J., Grinblatt, M., 1999. Do industries explain momentum? Journal of Finance 54, 1249-1290] industry momentum strategies. These profits do not show reversals in the long run. We find that the 52-week high is a better predictor of future returns than macroeconomic risk factors or the acquisition price. The individualism index, a proxy to the level of overconfidence, has no explanatory power to the variations of the 52-week high momentum profits across different markets. However, the profits are no longer significant in most markets once transaction costs are taken into account.

The economic growth of Central and Eastern Europe in comparative perspective, 1870–1989

Recurring changes in the institutional and ideological structures of Central and Eastern Europe have made it difficult to quantify and interpret its long-term economic development. This article provides the foundations for a unified picture of economic growth in the region since 1870. We construct a consistent time series of GDP per capita for the present day states of Central and Eastern Europe over the period 1870–1989, and then draw on the literature of the ‘new’ growth theory in economics to assess their long-term economic performance in the wider European setting.

Jackknife Estimator for Tracking Error Variance of Optimal Portfolios

We develop a jackknife estimator for the conditional variance of a minimum tracking error variance portfolio constructed using estimated covariances. We empirically evaluate the performance of our estimator using an optimal portfolio of 200 stocks that has the lowest tracking error with respect to the S&P 500 benchmark when three years of daily return data are used for estimating covariances. We find that our jackknife estimator provides more precise estimates and suffers less from in-sample optimism when compared to conventional estimators.

Are ex‐day dividend clientele effects dead? Dividend yield versus dividend size

We document new evidence that the ex-dividend day stock price behavior in the U.S. is inconsistent with the tax explanation in several aspects. We find that within a tick multiple, as dividend size increases, dividend yields increase, but the price-drop-to-dividend ratios decrease. For dividends that are less than a tick, there is no relation between price-drop-to-dividend ratio and dividend yield, and for dividends that are less than half a tick, the average price drop is larger than dividend amount. These facts contradict the tax explanation of the ex-dividend day phenomenon. But these patterns are qualitatively consistent with Dubofskys argument that the exchange rules on how to adjust the prices in the existing limit orders affect ex-day price drop behavior. The positive relation between the price-drop-to-dividend ratio and dividend yield is more pronounced as dividend size increases. We point out this is also qualitatively consistent with Dubofsky.

Assessing the Risk in Sample Minimum Risk Portfolios

We show that the in-sample estimate of the variance of a global minimum risk portfolio constructed using an estimated covariance matrix of returns will on average be strictly smaller than its true variance. Scaling the in-sample estimate upward by a standard degrees-of-freedom related factor or using the Bayes covariance matrix estimator can be inadequate; the correction is likely to be twice as large as the standard correction when returns are i.i.d. multivariate Normal. We develop a Jackknife-type estimator of the optimal portfolios variance that is valid when returns are i.i.d.; and a variation that may be better when returns exhibit volatility persistence. We empirically demonstrate the need to correct for in-sample optimism by considering an optimal portfolio of 200 stocks that has the lowest tracking error when the SP our correction, 1.24 percent; and the weighted Jackknife, 1.36 percent.

Are Short Sellers Informed? Evidence from the 2007–2008 Subprime Mortgage Crisis

This paper examines the short selling activities around financial firms’ announcements of asset write‐downs during the 2007–2008 subprime mortgage crisis. We find that short sellers accumulate short positions prior to write‐down announcements, and that stocks experience significantly negative returns around such announcements. These results suggest that the return predictability of short interests is due to short sellers’ informational advantage. Furthermore, we show that short sellers increase their positions significantly in the announcement month and keep increasing their positions afterward, suggesting the feedback effect of the disclosed write‐downs on financial firms’ existing exposures. The valuable information contained in the short interest should encourage regulators to mandate stock exchanges disclose short selling activities more frequently.