首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献,搜索用时 15 毫秒
1.
The problem of combining information from separate trials is a key consideration when performing a meta‐analysis or planning a multicentre trial. Although there is a considerable journal literature on meta‐analysis based on individual patient data (IPD), i.e. a one‐step IPD meta‐analysis, versus analysis based on summary data, i.e. a two‐step IPD meta‐analysis, recent articles in the medical literature indicate that there is still confusion and uncertainty as to the validity of an analysis based on aggregate data. In this study, we address one of the central statistical issues by considering the estimation of a linear function of the mean, based on linear models for summary data and for IPD. The summary data from a trial is assumed to comprise the best linear unbiased estimator, or maximum likelihood estimator of the parameter, along with its covariance matrix. The setup, which allows for the presence of random effects and covariates in the model, is quite general and includes many of the commonly employed models, for example, linear models with fixed treatment effects and fixed or random trial effects. For this general model, we derive a condition under which the one‐step and two‐step IPD meta‐analysis estimators coincide, extending earlier work considerably. The implications of this result for the specific models mentioned above are illustrated in detail, both theoretically and in terms of two real data sets, and the roles of balance and heterogeneity are highlighted. Our analysis also shows that when covariates are present, which is typically the case, the two estimators coincide only under extra simplifying assumptions, which are somewhat unrealistic in practice.  相似文献   

2.
3.
In this paper, our aim is to analyze geographical and temporal variability of disease incidence when spatio‐temporal count data have excess zeros. To that end, we consider random effects in zero‐inflated Poisson models to investigate geographical and temporal patterns of disease incidence. Spatio‐temporal models that employ conditionally autoregressive smoothing across the spatial dimension and B‐spline smoothing over the temporal dimension are proposed. The analysis of these complex models is computationally difficult from the frequentist perspective. On the other hand, the advent of the Markov chain Monte Carlo algorithm has made the Bayesian analysis of complex models computationally convenient. Recently developed data cloning method provides a frequentist approach to mixed models that is also computationally convenient. We propose to use data cloning, which yields to maximum likelihood estimation, to conduct frequentist analysis of zero‐inflated spatio‐temporal modeling of disease incidence. One of the advantages of the data cloning approach is that the prediction and corresponding standard errors (or prediction intervals) of smoothing disease incidence over space and time is easily obtained. We illustrate our approach using a real dataset of monthly children asthma visits to hospital in the province of Manitoba, Canada, during the period April 2006 to March 2010. Performance of our approach is also evaluated through a simulation study.  相似文献   

4.
Aims: Poor butanol tolerance of solventogenic stains directly limits their butanol production during industrial‐scale fermentation process. This study was performed to search for micro‐organisms possessing elevated tolerance to butanol. Methods and Results: Two strains, which displayed higher butanol tolerance compared to commonly used solventogenic Clostridium acetobutylicum, were isolated by evolution and screening strategies. Both strains were identified as lactic acid bacteria (LAB). On this basis, a LAB culture collection was tested for butanol tolerance, and 60% of the strains could grow at a butanol concentration of 2·5% (v/v). In addition, an isolated strain with superior butanol tolerance was transformed using a certain plasmid. Conclusions: The results indicate that many strains of LAB possessed inherent tolerance of butanol. Significance and Impact of the Study: This study suggests that LAB strains may be capable of producing butanol to elevated levels following suitable genetic manipulation.  相似文献   

5.
We consider a conceptual correspondence between the missing data setting, and joint modeling of longitudinal and time‐to‐event outcomes. Based on this, we formulate an extended shared random effects joint model. Based on this, we provide a characterization of missing at random, which is in line with that in the missing data setting. The ideas are illustrated using data from a study on liver cirrhosis, contrasting the new framework with conventional joint models.  相似文献   

6.
In the period from January 1981 to December 2010, 1068 small‐molecule new chemical entities (NCEs) were introduced, of which ca. 34% are either a natural product or a close analogue. While this metric reflects the impact natural products have played in delivering new chemical starting points (leads) for the pharmaceutical industry, it does not capture the decline this approach has suffered over the last 20 years as the high‐throughput screening (HTS) of pure compound libraries has become more popular. An impediment to natural‐product drug discovery in the HTS paradigm is the lack of a clear strategy that enables front‐loading of an extract or fraction's chemical constituents so that they are compliant with lead‐ and drug‐like chemical space. To address this imbalance, an approach based on lipophilicity, as measured by clog P has been developed that, together with advances being made in isolation and structural elucidation, can afford natural product leads in timelines compatible with pure compound screening.  相似文献   

7.
8.
Cystine‐knot peptides sharing a common fold but displaying a notably large diversity within the primary structure of flanking loops have shown great potential as scaffolds for the development of therapeutic and diagnostic agents. In this study, we demonstrated that the cystine‐knot peptide MCoTI‐II, a trypsin inhibitor from Momordica cochinchinensis, can be engineered to bind to cytotoxic T lymphocyte‐associated antigen 4 (CTLA‐4), an inhibitory receptor expressed by T lymphocytes, that has emerged as a target for the treatment of metastatic melanoma. Directed evolution was used to convert a cystine‐knot trypsin inhibitor into a CTLA‐4 binder by screening a library of variants using yeast surface display. A set of cystine‐knot peptides possessing dissociation constants in the micromolar range was obtained; the most potent variant was synthesized chemically. Successive conjugation with neutravidin, fusion to antibody Fc domain or the oligomerization domain of C4b binding protein resulted in oligovalent variants that possessed enhanced (up to 400‐fold) dissociation constants in the nanomolar range. Our data indicate that display of multiple knottin peptides on an oligomeric scaffold protein is a valid strategy to improve their functional affinity with ramifications for applications in diagnostics and therapy. Copyright © 2015 European Peptide Society and John Wiley & Sons, Ltd.  相似文献   

9.
10.
Multimode sensing was proposed for molecular screening and recognition of HER‐1 in whole blood. The tools used for molecular recognition were platforms based on nanostructured materials such as the complex of Mn(III) with meso‐tetra (4‐carboxyphenyl) porphyrin, and maltodextrin (dextrose equivalence between 4 and 7), immobilized in diamond paste, graphite paste or C60 fullerene paste. The identification of HER‐1 in whole‐blood samples, at molecular level, is performed using stochastic mode and is followed by the quantification of it using stochastic and differential pulse voltammetry modes. HER‐1 can be identified in the concentration range between 280 fg/ml and 4.86 ng/ml using stochastic mode, this making possible the early detection of cancers such as gastrointestinal, pancreatic and lung cancers. The recovery tests performed using whole‐blood samples proved that the platforms can be used for identification and quantification of HER‐1 with high sensitivity and reliability in such samples, these making them good molecular screening tools. Copyright © 2014 John Wiley & Sons, Ltd.  相似文献   

11.
Plant‐pathogenic fungi cause diseases to all major crop plants world‐wide and threaten global food security. Underpinning fungal diseases are virulence genes facilitating plant host colonization that often marks pathogenesis and crop failures, as well as an increase in staple food prices. Fungal molecular genetics is therefore the cornerstone to the sustainable prevention of disease outbreaks. Pathogenicity studies using mutant collections provide immense function‐based information regarding virulence genes of economically relevant fungi. These collections are rich in potential targets for existing and new biological control agents. They contribute to host resistance breeding against fungal pathogens and are instrumental in searching for novel resistance genes through the identification of fungal effectors. Therefore, functional analyses of mutant collections propel gene discovery and characterization, and may be incorporated into disease management strategies. In the light of these attributes, mutant collections enhance the development of practical solutions to confront modern agricultural constraints. Here, a critical review of mutant collections constructed by various laboratories during the past decade is provided. We used Magnaporthe oryzae and Fusarium graminearum studies to show how mutant screens contribute to bridge existing knowledge gaps in pathogenicity and fungal–host interactions.  相似文献   

12.
The minimal antibiotic options for carbapenemase‐producing Gram‐negative bacteria necessitate their rapid detection. A literature review of a variety of phenotypic and genotypic methods is presented. Advances in culture methods and screening media are still subject to long incubation hours. Biochemical methods have shorter turnaround times and higher sensitivities and specificities, but cannot differentiate between various types and variants. Spectrophotometric methods are cheap and efficient, but are uncommon in many clinical settings, while the MALDI‐TOF MS is promising for species identification, typing and resistance gene determination. Although next generation sequencing (NGS) technologies provide a better platform to detect, type and characterize carbapenem‐resistant bacteria, the different NGS platforms, the large computer memories and space needed to process and store genomic data and the nonuniformity in data analysis platforms are still a challenge. The sensitivities, specificities and turnaround times recorded in the various studies reviewed favours the use of the biochemical tests (Carba NP or Rapid Carb screen tests) for the detection of putative carbapenemase‐producing isolates. MALDI‐TOF MS and/or molecular methods like microarray, loop‐mediated isothermal amplification and real‐time multiplex PCR assays could be used for further characterization in a reference laboratory. NGS may be used for advanced epidemiological and molecular studies.  相似文献   

13.
Following the computation of a lattice energy landscape which predicted that there should be more stable, denser forms of (R)‐1‐phenylethylammonium‐(S)‐2‐phenylbutyrate, crystallizations from a range of solvents were performed to search for other polymorphs and investigate the possibility that the known P41 structure could be a hydrate. Extensive crystallization experiments from a wide range of solvents gave fine needles or microcrystalline samples. A redetermination of the P41 structure by powder X‐ray diffraction located all protons, and in conjunction with other experimental and computational evidence showed that the structure was anhydrous. Evidence for two additional forms was found as mixtures with form I. These include an orthorhombic form, possibly a Z′ = 3 polymorph, and another as yet unidentified form obtained as a minor component from dichloromethane solution. However, both these forms appear to be metastable with respect to form I (P41), which is therefore probably the most thermodynamically stable form that can be crystallized from solution under ambient conditions. This determination of the solid state behavior of the less readily crystallized member of the diastereomeric salt system (R)‐1‐phenylethylammonium‐(R/S)‐2‐phenylbutyrate provides a challenge to the theoretical modeling to explain its ideal resolution behavior. Chirality 2010. © 2009 Wiley‐Liss, Inc.  相似文献   

14.
15.
16.
17.
18.
Using resistant cultivars is the most sustainable and practical approach against plant diseases. Plant germplasm and breeding lines are selected and assayed against, usually, the most aggressive or virulent strains of a pathogen (e.g., fungus) that causes the disease. However, prolong storage of the pathogen in culture media could affect virulence that, consequently, also influence the outcome of the resistance assay. This study demonstrates that long‐term storage (at least a year) of Colletotrichum truncatum and C. scovillei, causal agents of pepper anthracnose, in potato dextrose agar (PDA) medium decreased the aggressiveness and virulence of the fungus in host‐pepper fruits. However, reintroduction of the pathogen to the host and isolation of the pathogen as the new inoculum, prior to inoculation assays, increased the virulence of the fungi. These findings suggest that re‐inoculation and re‐isolation of Colletotichum truncatum and C. scovillei that have been stored for at least 1 year in PDA medium are necessary when using fungal cultures in pathogenicity and plant resistance assays to achieve desirable, comparable and reliable results.  相似文献   

19.
20.
Analysis of longitudinal data with excessive zeros has gained increasing attention in recent years; however, current approaches to the analysis of longitudinal data with excessive zeros have primarily focused on balanced data. Dropouts are common in longitudinal studies; therefore, the analysis of the resulting unbalanced data is complicated by the missing mechanism. Our study is motivated by the analysis of longitudinal skin cancer count data presented by Greenberg, Baron, Stukel, Stevens, Mandel, Spencer, Elias, Lowe, Nierenberg, Bayrd, Vance, Freeman, Clendenning, Kwan, and the Skin Cancer Prevention Study Group[New England Journal of Medicine 323 , 789–795]. The data consist of a large number of zero responses (83% of the observations) as well as a substantial amount of dropout (about 52% of the observations). To account for both excessive zeros and dropout patterns, we propose a pattern‐mixture zero‐inflated model with compound Poisson random effects for the unbalanced longitudinal skin cancer data. We also incorporate an autoregressive of order 1 correlation structure in the model to capture longitudinal correlation of the count responses. A quasi‐likelihood approach has been developed in the estimation of our model. We illustrated the method with analysis of the longitudinal skin cancer data.  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号