ESTIMATING CENTRAL TENDENCY FROM UNCENSORED TRACE LEVEL MEASUREMENTS1

ABSTRACT: The statistical analysis of data which have trace level measurements has traditionally been a two-step process in which data are first censored using criteria based on measurement precision, and then analyzed with statistical methods for censored data. The process might be more informative if data were left uncensored. In this paper, information loss attributable to censoring and measurement noise are assessed by comparing the sample mean and median of uncensored measurements with a log regression mean and median based on censored data. Measurements are derived from lognormal parent distributions which have random variability characteristic of trace level measurement. The relative performance of estimators used with error-free samples and with samples having measurement noise can be explained by differences between the probability distributions of parents and measurements. Measurement introduces bias and dispersion and transforms lognormal parent distributions toward greater symmetry. Estimates using uncensored data are less biased and more accurate than the log regression mean and median when censoring exceeds about 50 percent, and are not much worse at any fraction censored. For data with many (80 percent) results below the limit of detection, bias may be quite severe.     

Monitoring the progress of attempts to reduce nutrient load and inputs of certain compounds in the north sea by 50%

During the winter period an inverse linear relation is found between the concentration of dissolved nutrients (phosphorus and nitrogen) and salinity in the Dutch coastal zone. This indicates a conservative behavior of these compounds from the river, through the estuary to the sea. During summer this relation is much more scattered because of biological processes. The physical and statistical properties of the relationship between salinity and the concentration of dissolved inorganic phosphorus and nitrogen are used to calculate when, where, and how many samples have to be taken in order to monitor a reduction of a compound accurately. It appeared that at any given salinity in the estuary and in the sea, the winter period is the most suitable season to detect a reduction of a given dissolved compound. The higher the salinity in the estuary, the more samples are required to prove the reduction significantly. A reduction of only 10% cannot even be demonstrated by field measurements during summer at salinities above 25. It is concluded that one cruise from the river to the sea, covering the salinities from 0 to 35 during the winter period, aimed at establishing the relationship between the concentration and salinity by taking samples at a salinity interval of for instance 1%, is sufficient to monitor a wide-ranging reduction of 10%–50% in both the fresh water and marine water. This program must be combined with a sampling at a salinity of 0, directed to determine the riverine temporal variability.     

国外环境监测管理的几点经验

从实验室布局,质量保证体系,实验室管理等方面介绍了国外环境监测管理的几点经验.     

工业污染源废水监测采样方法的改进

介绍了一种新型便携式采样器ＩＳＣＯ３７００，改进了手工采样方法，提高了污染源废水监测的质量。     

浅析室内环境污染检测与控制

分析了室内环境污染的由来、危害及内容,提出了对室内环境污染进行检测与控制,指出对室内环境污染的一些防治对策,表明室内空气污染必将引起广泛的关注和重视.     

浅析环境监测QC/QA过程的采样环节

对环境监测布点采样中的QC/QA以及由采样引起的误差来源做了简要分析,并对监测采样过程中QC/QA的操作提出了一些设想。     

沈阳经济技术开发区环境空气监测点位优化布点研究

在网格布点的基础上 ,采用群分析方法对沈阳经济技术开发区的环境空气监测数据进行科学论证和分析 ,得出最佳点位     

LRL-150型垃圾焚烧炉电视监控系统的设计

为了处理生活垃圾，减少城市污染，保护环境，经过多年研究，制造开发出日处理焚烧150m^3生活垃圾的垃圾焚烧炉。在高温作业下，为了使工作人员能清晰观察炉内燃烧状况。配备了适用于垃圾焚烧炉的高温电视自动化监控系统。通过它可以在监控室内直接观测、掌握焚烧炉运行情况，从而改善了工人的工作条件，提高了工作效率，经专家审定．认为可以推广。文中主要介绍了电视监控系统的结构、性能等情况。     

上海市TSP-Pb空气质量监测点位优化研究

通过对上海市23个站点的TSP-Pb监测数据的分析,发现TSP与Pb之间具有良好的相关性.因此考虑只用TSP数据对站点进行优化.通过对各站点TSP数据的聚类分析,将所有站点分为6类,然后对包含多个站点的类,将类中各站点与本类均值一同进行再聚类,选取代表站点.最终选取6个站点作为上海市TSP-Pb污染水平的代表站点.优化前后的对比分析表明,优化结果合理,信息损失量小.