In diagnostic trials, clustered data are obtained when several subunits of the same patient are observed. Within-cluster correlations need to be taken into account when analyzing such clustered data. A nonparametric m...In diagnostic trials, clustered data are obtained when several subunits of the same patient are observed. Within-cluster correlations need to be taken into account when analyzing such clustered data. A nonparametric method has been proposed by Obuchowski (1997) to estimate the Receiver Operating Characteristic curve area (AUC) for such clustered data. However, Obuchowski’s estimator gives equal weight to all pairwise rankings within and between cluster. In this paper, we modify Obuchowski’s estimate by allowing weights for the pairwise rankings vary across clusters. We consider the optimal weights for estimating one AUC as well as two AUCs’ difference. Our results in this paper show that the optimal weights depends on not only the within-patient correlation but also the proportion of patients that have both unaffected and affected units. More importantly, we show that the loss of efficiency using equal weight instead of our optimal weights can be severe when there is a large within-cluster correlation and the proportion of patients that have both unaffected and affected units is small.展开更多
Ecological assembly rules in evergreen broad-leaved forest are far from clear understanding.Spatial dispersion of individuals in a species is central in ecological theory.We analyzed the spatial patterns as well as as...Ecological assembly rules in evergreen broad-leaved forest are far from clear understanding.Spatial dispersion of individuals in a species is central in ecological theory.We analyzed the spatial patterns as well as associations between adult and juvenile of each tree species in a 5-ha subtropical evergreen broad-leaved forest plot in eastern China.Out of the 74 species occurring with more than 10 individuals,88.4%of these species are aggregated.Most of them are aggregated from small to large scales.Spatial distributions of some species corre-spond with topography.Many bad dispersed species in the Baishanzu exhibit a highly aggregated distribution at small scales.These suggest that environmental heterogeneity and/or dispersal limitation may be the most important mechanisms that control the distribution patterns of these species.Our observations of the aggregations of abundant species basically support the hypothesis that dispersal limitation decreases as the number of reproductive trees increases.The rest species are randomly distributed,with less than 10 individuals.For most common species,spatial aggregation is weaker in larger diameter classes,and the distance between adults is larger than that between juveniles and adults,suggesting that density-dependence works on loosing aggregation and excluding conspecific juveniles away from adult trees.However,the density-dependent effect is not strong enough to eliminate all seedlings near adult trees and to result in a regular distribution of trees;thus the density dependence is usually masked by the refuge effect.展开更多
文摘In diagnostic trials, clustered data are obtained when several subunits of the same patient are observed. Within-cluster correlations need to be taken into account when analyzing such clustered data. A nonparametric method has been proposed by Obuchowski (1997) to estimate the Receiver Operating Characteristic curve area (AUC) for such clustered data. However, Obuchowski’s estimator gives equal weight to all pairwise rankings within and between cluster. In this paper, we modify Obuchowski’s estimate by allowing weights for the pairwise rankings vary across clusters. We consider the optimal weights for estimating one AUC as well as two AUCs’ difference. Our results in this paper show that the optimal weights depends on not only the within-patient correlation but also the proportion of patients that have both unaffected and affected units. More importantly, we show that the loss of efficiency using equal weight instead of our optimal weights can be severe when there is a large within-cluster correlation and the proportion of patients that have both unaffected and affected units is small.
文摘Ecological assembly rules in evergreen broad-leaved forest are far from clear understanding.Spatial dispersion of individuals in a species is central in ecological theory.We analyzed the spatial patterns as well as associations between adult and juvenile of each tree species in a 5-ha subtropical evergreen broad-leaved forest plot in eastern China.Out of the 74 species occurring with more than 10 individuals,88.4%of these species are aggregated.Most of them are aggregated from small to large scales.Spatial distributions of some species corre-spond with topography.Many bad dispersed species in the Baishanzu exhibit a highly aggregated distribution at small scales.These suggest that environmental heterogeneity and/or dispersal limitation may be the most important mechanisms that control the distribution patterns of these species.Our observations of the aggregations of abundant species basically support the hypothesis that dispersal limitation decreases as the number of reproductive trees increases.The rest species are randomly distributed,with less than 10 individuals.For most common species,spatial aggregation is weaker in larger diameter classes,and the distance between adults is larger than that between juveniles and adults,suggesting that density-dependence works on loosing aggregation and excluding conspecific juveniles away from adult trees.However,the density-dependent effect is not strong enough to eliminate all seedlings near adult trees and to result in a regular distribution of trees;thus the density dependence is usually masked by the refuge effect.
