STR遗传多态性调查中样本量和采样方式选择初探

目的对STR基因座进行遗传学调查时样本量大小和采样方式进行考察分析。方法用DNA Typer~(TM)19试剂盒对血卡样本进行直扩,ABI3730型遗传分析仪电泳检测,Genemapper ID v3.2软件进行等位基因分型,根据公式计算群体遗传学参数。计算样本量在50、100、150、200、250、300、350、400、450、500十个水平上遗传多态性水平,并评估与总体的差异。计算常见的四种采样方法(整群采样、随机采样、系统采样、分层采样)所抽取样本遗传多态性水平,并评估抽样误差。结果 18个STR基因座在河北地区16 058份随机样本共检出317种等位基因,基因频率分布在3.114e-5~0.515。18个STR基因座在河北群体水平上PM为6.04e-14,TDP为0.999 999 999 999 94,CEP为0.999 999 987 514 828。PM值、CEP值在样本量大于200后中值基本稳定。四种抽样方法的抽样误差大小是:整群抽样≥单纯随机抽样≥系统抽样≥分层抽样。结论 DNATyper~(TM)19试剂盒的18个STR基因座在河北地区具有较高的遗传多态性水平。在小范围内进行人群频率调查时,可随机选择200份样本代表当地遗传多态性水平。在相对较大范围或遗传背景复杂的区域,可采用分层抽样的方法降低抽样误差。     

Bayesian adaptive approach to estimating sample sizes for seizures of illicit drugs

A considerable amount of discussion can be found in the forensics literature about the issue of using statistical sampling to obtain for chemical analyses an appropriate subset of units from a police seizure suspected to contain illicit material. Use of the Bayesian paradigm has been suggested as the most suitable statistical approach to solving the question of how large a sample needs to be to ensure legally and practically acceptable purposes. Here, we introduce a hypergeometric sampling model combined with a specific prior distribution for the homogeneity of the seizure, where a parameter for the analyst's expectation of homogeneity (α) is included. Our results show how an adaptive approach to sampling can minimize the practical efforts needed in the laboratory analyses, as the model allows the scientist to decide sequentially how to proceed, while maintaining a sufficiently high confidence in the conclusions.     

Comparison of Frequentist Methods for Estimating the Total Weight of Consignments of Drugs

Abstract: A topic in forensic statistics is the estimation of the total weight of consignments of drugs based on subsamples of which a certain fraction may not contain drugs at all. The frequentist approach to this concentrates on obtaining confidence intervals for the total weight, based on estimation of the fraction of drugs and the mean and variance of the weights of drug units. The current study shows that the resulting confidence intervals are basically unreliable, since they are based on an underestimation of the variation of the underlying statistical process. Two alternatives are given that yield asymptotically correct results. These are not reliable for small subsamples either, though, because of the inherent multimodal behaviour of the sample mean. In cases where a relatively large fraction of the consignment contains no drugs, the confidence intervals reported in the literature should not be used in practice.     

Efficiency evaluation of a police operation to fight the drug plague: distribution unit weight as an objective index

Lod, a city near Tel-Aviv, is considered the main drug distribution center in Israel. A major police undercover operation in Lod, lasting close to a year, was terminated in May 2003. The success or failure of such an operation is frequently measured by the number of arrests made, the hierarchical level of the dealers arrested, the number of drug stations closed down, and the decrease in heroin seizures following the operation. In this work we suggest using an additional parameter, which has a scientific, objective basis, namely, comparing the changes in the average user weight unit ("dose") before and after the operation. We found that prior to the operation the average weight per unit was 1.1 g. Three months after the operation terminated the average weight per unit had decreased to 0.8 g and remained there for at least 4 months before rising again.     

Utility of immunoassay in drug screening in skeletal tissues: sampling considerations in detection of ketamine exposure in femoral bone and bone marrow following acute administration using ELISA

Detection of ketamine exposure in skeletal tissues by automated enzyme-linked immunosorbent assay (ELISA) and gas chromatography with electron capture detection (GC-ECD) is described. Rats (n = 18) received 0, 15, 30, or 75 mg/kg ketamine hydrochloride acutely (i.p.), and were euthanized within 15 min or 1 h. Ketamine was extracted from ground femoral bone by methanolic incubation followed by liquid-liquid extraction (LLE), while marrow was homogenized in alkaline solution, and then underwent LLE. Extracts were analyzed by ELISA, and subsequently by GC-ECD following derivatization with trifluoroacetic acid anhydride. The effect of tissue type (i.e., diaphyseal bone vs. epiphyseal bone vs. bone marrow) on the immunoassay response was examined through determination of binary classification test sensitivity (S) and measurement of the relative decrease in absorbance (%DA, drug-positive tissues vs. drug-free controls) in each tissue type. The %DA varied significantly between different tissues examined under a given dose condition, and generally decreased in the order marrow > epiphyseal bone > diaphyseal bone, at all dose levels examined. Measured S values for marrow, epiphyseal bone, and diaphyseal bone were 100%, 77%, and 23%, respectively (75 mg/kg dose). These results suggest that the type of skeletal tissue sampled and position sampled within a given bone (diaphyses vs. epiphyses) are important parameters in drug screening of skeletal tissues.     

Particle size analysis of soils under simulated scene of crime conditions: the interest of multivariate analyses

Two simulated scenes of crime have been studied. Soil traces adhering to boots, sport shoes and tissues have been compared with control samples using particle size analysis. Comparisons of percentage of particles per class interval and multivariate analyses were used to determine how the size distribution of each suspect sample varied compared to the original distribution. A loss of coarse fractions characterized most of the suspect samples even though they were derived from soils having different distributional patterns. However, the differences between control and suspect samples were found to be more important for soils dominated by extreme size classes than for soils rich in medium size particles. The differences among soil samples from the same source were found to be restricted to the extreme size classes. The finding of significant differences within size classes ranging from 1 to 0.063 mm should be then considered as a dissimilarity sign between trace sample and hypothetic control soil.     

Quantifying Uncertainty in Estimations of the Total Weight of Drugs in Groups of Complex Matrices: Using the Welch–Satterthwaite Equation

In this paper, a method is described to quantify estimations of the total amount of drugs in groups of seized items, including quantification of the precision. Previous work on this topic was based on the assumptions of normally distributed measurements and grouping of items with a common relative standard deviation. In practice, these assumptions are often violated, for example, for data with point masses at 0, or if certain items in a group have a very high standard deviation. The method described in this paper is based on work by Welch and Satterthwaite and does not assume constant relative standard deviations. Case examples are described for which the method is applied, and simulation studies are carried out for which both methods are applied. In the cases, both methods perform reasonably well. If the assumption of common relative standard deviations clearly does not apply, it is advised to use the method described.     

The Spatial Distribution of Postblast RDX Residue: Forensic Implications

Locating exactly where trace explosive residue samples should be sought during sample collection at bomb scenes is not specified in the published literature or guidelines; in this area, it is generally acknowledged that forensic practices are based on tradition rather than evidence. This study investigated patterns in the spatial distribution of postblast 1,3,5‐trinitro‐1,3,5‐triazocyclohexane residue from a series of unconfined detonations, over a range of sampling sites, and at two different detonation heights. The amount of residue recovered from the sites decreased as a function of distance from the center of the explosion. [Correction added after online publication 27 December 2012: In the preceding sentence, “increased” was corrected to “decreased” to agree with the conclusion of the article.] As the height of the detonations increased, more residues were found from all sampling sites. The findings of this empirical study have a number of important practical implications including determining where residue samples are best sought at crime scenes.