Temporary Components of Stock Prices: New Univariate Results
提出一个可估计的下界，衡量股票价格中可预测成分对总收益方差的贡献，发现纽约证券交易所股票在1947-1986年间该比例最高为10%，1926-1946年间最高为25%。
While there is growing evidence that stock prices do not follow pure random walks, the degree of existence of temporary components in stock prices is not well known. Modeling stock prices as the sum of a random walk and a general stationary (predictable) component, the paper proposes an estimable lower bound on the proportion of total stock return variance caused by the predictable component. Contrary to the absolute value of the first-order autocorrelation coefficient estimates of Fama and French (1988a), this lower bound reasonably estimates the true variance proportion in finite samples also when the temporary compo? nent does not follow a first-order autoregressive process. The estimated mean values of the lower bound reach a maximum of 10 percent for the equal-weighted market portfolio of NYSE stocks over the post-war period 1947-1986, while the maximum is 25 percent for the pre-war period 1926-1946. The value-weighted market portfolio exhibits generally smaller variance proportion estimates. The pure random walk hypothesis is also reexamined using a standard variance ratio statistic extended to multiple return horizons.