Statistical power of non-parametric tests: a quick guide for designing sampling strategies

The importance of considering statistical power in marine pollution studies is unequivocal. However, the vast majority of ecological literature on power analysis focuses on parametric rather than non-parametric tests. This note describes a Monte Carlo simulation method for estimating the power of non-parametric tests. The method is illustrated using ordinal data.     

QK系列多功能取样钻机的研制

介绍了QK系列多功能取样钻机的类型、基本结构以及性能特点。该系列钻机在实际应用中,体现了结构设计合理先进,较好地突出了＂轻便、快捷、高效＂的特点与优势。     

反射波法在断裂桩检测中的应用

结合工程实践,分析了断裂桩的形成原因及检测原理,探讨了检波器和采样滤波频率对反射波曲线的影响,断裂桩在反射波法中的曲线特征及计算方法.     

小波变换及其应用

小波变换是近年来发展起来的一门新的数学分支.它适宜于分析研究信号的局部性质,对于图象数据压缩、奇性检测、非卷积型算子的化简及取样插值定理等方面都有着重要的应用,在地震勘探、大气湍流、语声合成、图象处理等许多领域中都有着广泛应用的前景.本文介绍了小波变换及其正交基的基本概念,并对它的一些重要应用作了概括的介绍.     

Slope profile variability in first-order drainage basins

If the three-dimensional form of a hillslope is regarded as an infinite population of two-dimensional slope profiles, values for attributes of the form of the hillslope can be determined by sampling and estimating the means of the attributes of the population of slope profiles. Twelve attributes are examined in seven first-order drainage basins located on the South Downs in East Sussex to determine necessary sample sizes. Results show that if spatial randomness is assumed the size of sample required varies considerably from one attribute to another but is commonly very large. Of four alternatives to spatial randomness that are tested, consideration of both hillslope plan form and valley asymmetry are shown not to lead to reduction in sample size. However, results do suggest that maximum slope, percentage of profile length in the range 5° to < 10°, and mean angular difference between adjacent slope segments may all be dependent upon distance of the slope profile down valley. Furthermore, initial tests of the effect of spatial autocorrelation suggest that for some attributes consideration needs to be given to this factor when determining sampling strategies.     

Accuracy and precision of methods for estimating river loads

River loads often have to be estimated from continuous discharge data but relatively infrequent sampling of sediment, solute, or pollutant concentrations. Two standard ways of doing this are to multiply mean concentration by mean discharge, and to use a rating curve to predict unmeasured concentrations. Both methods are known from previous empirical studies to underestimate true load. Statistical considerations explain these biases and yield correction factors which can be used to obtain unbiased estimates of load. Simulation experiments with normally-distributed scatter about log-linear trends, and sampling experiments using a natural data set, show that the corrected rating curve method has lower sampling variability than other unbiased methods based on average instantaneous load and is thus the recommended procedure when the rating plot is of the assumed form. The precision of all methods increases with sample size and decreases with increasing rating-curve slope and scatter.     

Effects of time under rain,sampling technique and transport of samples on size distributions of water-stable aggregates

Two methods for sampling aggregates in the soil surface under simulated rain were compared using two soil types. Results showed that aggregate size distributions obtained by spatula sampling were not significantly different from those obtained using rings buried in the soil surface, provided both were sampled to the same depth. The effect of transporting samples over a distance of 60 km was non-significant when samples were placed in bottles half-filled with rainwater and transported in an upright position. The per cent aggregates > 0.125 mm was found to be the most suitable index of aggregate stability for both soils.     

Uncertainty reduction and sampling efficiency in slope designs using 3D conditional random fields

A method of combining 3D Kriging for geotechnical sampling schemes with an existing random field generator is presented and validated. Conditional random fields of soil heterogeneity are then linked with finite elements, within a Monte Carlo framework, to investigate optimum sampling locations and the cost-effective design of a slope. The results clearly demonstrate the potential of 3D conditional simulation in directing exploration programmes and designing cost-saving structures; that is, by reducing uncertainty and improving the confidence in a project’s success. Moreover, for the problems analysed, an optimal sampling distance of half the horizontal scale of fluctuation was identified.     

采伐剩余物的线截抽样技术理论研究

研究了采伐剩余物调查的线截抽样原理和方法。在应用中既可使用单条样线进行估计,也可以使用多条样线进行估计。在计算机上应用线截抽样估计原理模拟了采伐剩余物的调查情况,结果表明在8次模拟中7次实际精度达到80％以上,有一半在90％以上。     

Sampling efficiency of vertical array aeolian sand traps

Previous investigations have indicated that the sampling efficiency of aeolian sand traps is low and varies greatly in the near-bed region. Outside this region, the efficiency tends to be consistently higher for all types of trap. An evaluation was carried out to compare the sampling efficiency of different types of aeolian sand trap based on the comparison of the “actual” and the measured sand mass flux profiles, with emphasis on the single-tube vertical array trap, conventional array trap, and step-like array trap. A simple formula is proposed to express the actual vertical profile of sand mass flux, which has been validated with the unique data obtained with an isokinetic trap by [Sedimentology 45 (1998) 789]. Using the experimental data collected by the present authors and those by other investigators, sampling efficiencies of three types of trap are examined in terms of the frequency distribution of all the samples. For the single-tube traps, the sampling efficiency varies from 65% to 95%, with a mode at 75%. For both the conventional array and step-like array traps, sampling efficiencies range from 15% to 85%, with the modal frequencies at 35% and 75%, respectively. This review seems to suggest that the peak frequency with higher sampling efficiency coincides with the maximum sand-grain Reynolds number.