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931.
上海股票市场"规模效应"的实证研究及原因探析 总被引:7,自引:0,他引:7
"规模效应"是股票市场上一种重要的异常收益现象,反映了股票收益与公司规模之间的反向关系.选择沪市所有A股为样本,以流通市值来衡量公司的规模,1997年1月至2002年12月间沪市股票(组合)的平均收益与公司规模之间具有明显的负相关关系.通过逐月横截面回归分析进一步检验了公司规模对股票收益的解释能力,证明了沪市"规模效应"的稳健性.上海股票市场"规模效应"产生的原因主要在于市场操纵和公司购并行为. 相似文献
932.
A panel study consists of individuals who have data collected at periodic follow-up visits or pre-specified time points following entry into the study. The objective of this paper is to consider design issues in a panel study when the response variable is the stage of disease, and with focus on the transition intensities. Important design issues include the choice of the time interval between follow-up visits and sample size considerations. We study the effects of time intervals between follow-up visits on the precision of the transition intensities estimators. We also consider the power of statistical tests on the ratio of transition intensities. Discussion is extended to incorporate heterogeneity in the population in which frailty is introduced to describe subject-specific transition intensities. 相似文献
933.
934.
Koji Tsuchida 《Researches on Population Ecology》1991,33(2):361-366
Summary Females in hibernation site (FH) were significantly larger than foundresses (FO), suggesting that larger FH were less successful
in founding their nest. Body size of FO did not correlate with two nest characters, i.e., the number of cells at pupal stage
(CP) and the timing of nest foundation (DP). However, CP negatively correlated with DP in 2 of 3 studied years, suggesting
that foundress who founded her nest earlier than others tended to construct larger number of cells irrespective of her body
size. 相似文献
935.
本文介绍了国内外光学全散射法测粒技术的发展过程、存在的主要问题,提出一改进后的新方法:用多波长全散射法测量颗粒粒径分布。该方法不仅可消除全散射法测量结果有多值性的缺陷,而且可以直接获得颗粒的粒径分布,测试粒径范围也有所扩充(0.03~8μm)。文章还给出了用独立及非独立模式的两种算法,求解粒径分布的数值模拟结果以及对聚苯乙烯标准颗粒的实测结果。计算和实测的结果令人满意。 相似文献
936.
The condition of fixed sample size is essential for the existence of a Sen-Yates-Grundy form variance and its design unbiased estimator, in the problem of estimating the mean, variance and covariance of a finite population. 相似文献
937.
Kai Fun Yu 《Revue canadienne de statistique》1989,17(4):377-389
Suppose it is desired to obtain a large number Ns of items for which individual counting is impractical, but one can demand a batch to weigh at least w units so that the number of items N in the batch may be close to the desired number Ns. If the items have mean weight ωTH, it is reasonable to have w equal to ωTHNs when ωTH is known. When ωTH is unknown, one can take a sample of size n, not bigger than Ns, estimate ωTH by a good estimator ωn, and set w equal to ωnNs. Let Rn = Kp2N2s/n + Ksn be a measure of loss, where Ke and Ks are the coefficients representing the cost of the error in estimation and the cost of the sampling respectively, and p is the coefficient of variation for the weight of the items. If one determines the sample size to be the integer closest to pCNs when p is known, where C is (Ke/Ks)1/2, then Rn will be minimized. If p is unknown, a simple sequential procedure is proposed for which the average sample number is shown to be asymptotically equal to the optimal fixed sample size. When the weights are assumed to have a gamma distribution given ω and ω has a prior inverted gamma distribution, the optimal sample size can be found to be the nonnegative integer closest to pCNs + p2A(pC – 1), where A is a known constant given in the prior distribution. 相似文献
938.
We studied seasonal changes in the larval population structure, adult size, and autogeny (egg production without a bloodmeal)
of the mosquitoAedes togoi on the seacoast of northern Kyushu, Japan. The effects of temperature, photoperiod and food conditions on larval development,
adult size and autogeny were studied in the laboratory.Aedes togoi overwintered in both the egg and larval stages and was multivoltine. Adult size was greater in spring and autumn and smaller
in summer. Autogeny occurred in spring and autumn but not in summer. Autogenous females were larger than anautogenous females,
and larger autogenous females produced more eggs. Laboratory experiments showed that autogeny was promoted under low temperature
and short-day conditions which corresponded with spring/autumn conditions in the field. Experimental food limitation in the
larval stage greatly reduced adult size and autogenous egg production. In the field, large inter-pool variations in adult
size and autogeny rate probably resulted from variations in nutrient and crowding conditions. Seasonal autogeny ofAedes togoi was discussed in terms of life-history strategy under fluctuating environmental conditions where relative advantages of autogenous
and anautogenous reproduction alternate seasonally. 相似文献
939.
Alois Honk 《Researches on Population Ecology》1995,37(1):111-118
Functional alary polymorphisms have been studied rather extensively in several insect species. This review article deals with
factors controlling wing polymorphism in a flightless species,Pyrrhocoris apterus (L.), and discusses its adaptive significance and mechanisms for their persistence under natural conditions. The macropterous
morph is determined by a recessive allele whose penetrance depends on photoperiod and temperature. Natural populations of
this species contain a small fraction of flightless macropters. The disadvantages of being a macropter (increase of development
time, decrease of fecundity) are minimal, while the benefit may consist in the tendency to prereproductive arrest of ovarian
development in teneral females. It prevents establishing a second generation which would mostly die during the next winter.
The mechanism of alary morph regulation may be an ancestral trait linkingP. apterus with other polymorphic Heteroptera, while its decreased penetrance may be a derivative character. Variation in fitness due
to alary morphs is small compared to the one associated with differences in body size. The latter is environmentally determined,
and not linked to the genetic basis of wing polymorphism. In the “mosaic” of phenotypes of various size the significance of
the genetic macroptery may be close to neutral. 相似文献
940.
Yoichi Shirai 《Researches on Population Ecology》1995,37(2):269-277
Plutella xylostella in the temperate zone shows a clear seasonal change in adult body size. In the laboratory, large and small moths were produced
during immature stages at 15°C and 25°C, respectively. These moths were then used to evaluate longevity, age-specific flight
ability, flight ability of mated and unmated females, and the influence of flight experience on the subsequent reproductive
success. The large moths lived longer and displayed a greater flight ability over 3 weeks. Irrespective of body size, unmated
females flew for a longer time than mated females, and flight experience affected their subsequent reproductive success. Females
of both sizes mated and laid eggs soon after emergence, without any obvious pre-reproductive period. More flight experience
did not delay oviposition, but did reduce egg production. It is likely that large moths with a longer adult life span and
greater flight ability are better fitted for long-distance flight and more fecund than small ones. These experimental results
may explain why long-distance migration ofP. xylostella is mostly seen during cool seasons, when relatively large moths with long forewing appear in the field. 相似文献