共查询到20条相似文献，搜索用时 0 毫秒
1.
Publication bias in meta-analysis is usually modeled in terms of an accept/reject selection procedure in which the selected studies are the \"published\" studies and the rejected studies are the \"unpublished\" studies. One possible selection mechanism is to suppose that only studies that report an estimated treatment effect exceeding (or falling short of) some threshold are accepted. We show that, with appropriate choice of thresholds, this attains the maximum bias among all selection mechanisms in which the probability of selection increases with study size. It is impossible to estimate the selection mechanism from the observed studies alone: this result leads to a \"worst-case\" sensitivity analysis for publication bias, which is remarkably easy to implement in practice. The method is illustrated using data on the effectiveness of prophylactic corticosteroids. 相似文献
2.
In a meta-analysis of randomized trials of the effects of dietary sodium interventions on blood pressure, we found substantial heterogeneity among the studies. We were interested in evaluating whether measurement error, known to be a problem for dietary sodium measures, publication bias, or confounding factors could be responsible for the heterogeneity. A measurement error correction was developed that corrects both the slope and the intercept and takes into account the sample size of each study and the number of measurements taken on an individual. The measurement error correction had a minimal effect on the estimates, although it performed well in simulated data. A smoothed scatter plot was used to assess publication bias. Metaregressions provide a convenient way to jointly assess the effects of several factors, but care must be taken to fit an appropriate model. 相似文献
3.
Publication bias of the results of medical studies can invalidate evidence-based medicine. The existing methodology for modeling this essentially relies upon the symmetry of the funnel plot. We present a new method of modeling publication bias that uses this information plus the impact factors of the publishing journals. A simple model of the publication process enables the estimation of bias-corrected intervention effects. The procedure is illustrated using a meta-analysis of the effectiveness of single-dose oral aspirin for acute pain, and results are also obtained for five other meta-analyses. The method enables the fitting of a wide range of models and is considered more flexible than other ways of compensating for publication bias. The model also provides the basis of a statistical test for the existence of publication bias. Use of the new methodology to supplement existing methods is recommended, in the context of a sensitivity analysis. 相似文献
4.
The conservation literature provides an important resource for conservation practice and reflects key areas of current research interest, but not necessarily those of highest priority, in the field of conservation science. In one of the most comprehensive studies of its kind, this paper evaluates 4388 articles from three leading conservation journals with different impact factors (Biological Conservation, Conservation Letters, and Oryx) for the period 2010–2019. The analysis shows that previously identified trends still hold true in many instances, including a continuing taxonomic bias towards vertebrates and particularly in favour of mammals. Geographical biases also persist, favouring Europe and North America with respect to where research is undertaken, who writes about it, and how it is funded, although publication of research conducted in Africa is increasing. It is difficult to evaluate changes in author gender outside the decade studied as this topic has not been widely considered in previous studies, but parity in authorship has not yet been achieved in any of the journals examined. To overcome the biases identified, researchers, editors, and funding organisations collectively still need to do more to redirect conservation efforts where they are most urgently needed and encourage more women and more researchers from lower income countries to participate in research by helping to remove the barriers that currently limit them. 相似文献
5.
We study recently developed nonparametric methods for estimating the number of missing studies that might exist in a meta-analysis and the effect that these studies might have had on its outcome. These are simple rank-based data augmentation techniques, which formalize the use of funnel plots. We show that they provide effective and relatively powerful tests for evaluating the existence of such publication bias. After adjusting for missing studies, we find that the point estimate of the overall effect size is approximately correct and coverage of the effect size confidence intervals is substantially improved, in many cases recovering the nominal confidence levels entirely. We illustrate the trim and fill method on existing meta-analyses of studies in clinical trials and psychometrics. 相似文献
6.
We study publication bias in meta-analysis by supposing there is a population (y, sigma) of studies which give treatment effect estimates y approximately N(theta, sigma(2)). A selection function describes the probability that each study is selected for review. The overall estimate of theta depends on the studies selected, and hence on the (unknown) selection function. Our previous paper, Copas and Jackson (2004, Biometrics 60, 146-153), studied the maximum bias over all possible selection functions which satisfy the weak condition that large studies (small sigma) are as likely, or more likely, to be selected than small studies (large sigma). This led to a worst-case sensitivity analysis, controlling for the overall fraction of studies selected. However, no account was taken of the effect of selection on the uncertainty in estimation. This article extends the previous work by finding corresponding confidence intervals and P-values, and hence a new sensitivity analysis for publication bias. Two examples are discussed. 相似文献
7.
L. P. Jones W. A. Compton 《TAG. Theoretical and applied genetics. Theoretische und angewandte Genetik》1985,70(3):318-321
Summary The use of several S1 individuals to represent an S0 individual permits the use of a Design II mating scheme for plants with only one pistillate flower per plant. Estimates of additive (VA) and dominance (VD) variance from this mating scheme will be biased upwards, when a small number (10) of individuals of each S1 line are used. This bias can be computed, and the additive and dominance estimates can be corrected. Of particular interest is the observation that the additive genetic variance contributes to bias in estimates of VD. When S0 plants are non inbred and their selfedprogeny (S1 lines) are used to represent them in developing families for use in the Design II, where m1 is the number of individuals used to represent an S1 line in developing half sib-families and m2 is the number of individuals used to represent the S1 line in making up full sib-families. For example, in a 3×3 Design II, with about 10 individuals used to represent each S1 line in each cross, m2 = 10 and m1 = 30. When m1 = m2 = 1, and Joint contribution from Department of Agronomy, University of Nebraska 68583, and the S. S. Cameron Laboratory, Werribee, Victoria 3030, Australia. Published as paper No. 7395, Journal Series 相似文献
8.
Evidence synthesis, both qualitatively and quantitatively through meta-analysis, is central to the development of evidence-based medicine. Unfortunately, meta-analysis is often complicated by the suspicion that the available studies represent a biased subset of the evidence, possibly due to publication bias or other systematically different effects in small studies. A number of statistical methods have been proposed to address this, among which the trim-and-fill method and the Copas selection model are two of the most widely discussed. However, both methods have drawbacks: the trim-and-fill method is based on strong assumptions about the symmetry of the funnel plot; the Copas selection model is less accessible to systematic reviewers, and sometimes encounters estimation problems. In this article, we adopt a logistic selection model, and show how treatment effects can be rapidly estimated via multiple imputation. Specifically, we impute studies under a missing at random assumption, and then reweight to obtain estimates under nonrandom selection. Our proposal is computationally straightforward. It allows users to increase selection while monitoring the extent of remaining funnel plot asymmetry, and also visualize the results using the funnel plot. We illustrate our approach using a small meta-analysis of benign prostatic hyperplasia. 相似文献
9.
Meta-analysis,funnel plots and sensitivity analysis 总被引：3，自引：0，他引：3
Publication bias is a major problem, perhaps the major problem, in meta-analysis (or systematic reviews). Small studies are more likely to be published if their results are 'significant' than if their results are negative or inconclusive, and so the studies available for review are biased in favour of those with positive outcomes. Correcting for this bias is not possible without making untestable assumptions. In this paper, a sensitivity analysis is suggested which is based on fitting a model to the funnel plot. Some examples are discussed. 相似文献
10.
11.
M. Singh 《Biometrical journal. Biometrische Zeitschrift》1979,21(7):645-647
This paper advocates a procedure of obtaining an estimator (as a linear combination of a set of ratio estimators) whose bias can be reduced to a desired degree of sample size without any additional loss in efficiency. 相似文献
12.
《Animal : an international journal of animal bioscience》2019,13(10):2277-2288
Direct measurement of individual animal dry matter intake (DMI) remains a fundamental challenge to assessing dairy feed efficiency (FE). Digesta marker, is currently the most used indirect technique for estimating DMI in production animals. In this meta-analysis we evaluated the performance of marker-based estimates against direct or observed measurements and developed equations for the prediction of FE (g energy-corrected milk (ECM)/kg DMI). Data were taken from 29 change-over studies consisting of 416 cow-within period observations. Most studies used more than one digesta marker. So, for each observed measurement of DMI, faecal dry matter output (FDMO) and apparent total tract dry matter digestibility (DMD), there was one or more corresponding marker estimate. There were 924, 409 and 846 observations for estimated FDMO (eFDMO), estimated apparent total tract DMD (eDMD) and estimated DMI (eDMI), respectively. The experimental diets were based mainly on grass silage, with soya bean or rapeseed meal as protein supplements and cereal grains or by-products as energy supplements. Across all diets, average forage to concentrate ratio on a dry matter (DM) basis was 59 : 41. Variance component and repeatability estimates of observed and marker estimations were determined using random factors in mixed procedures of SAS. Between-cow CV in observed FDMO, DMD and DMI was, 10.3, 1.69 and 8.04, respectively. Overall, the repeatability estimates of observed variables were greater than their corresponding marker-based estimates of repeatability. Regression of observed measurements on marker-based estimates gave good relationships (R2=0.87, 0.68, 0.74 and 0.74, relative prediction error =10.9%, 6.5%, 15.4% and 18.7%for FDMO, DMD, DMI and FE predictions, respectively). Despite this, the mean and slope biases were statistically significant (P<0.001) for all regressions. More than half of the errors in all regressions were due to mean and slope biases (52.4% 87.4%, 82.9% and 85.8% for FDMO, DMD, DMI and FE, respectively), whereas the contributions of random errors were small. Based on residual variance, the best model for predicting FE developed from the dataset was FE (g ECM/kg DMI)=1179(±54.1) +38.2(±2.05)×ECM(kg/day)−0.64(±0.051)×BW (kg)−75.6(±4.39)×eFDMO (kg/day). Although eDMD was positively related to FE, it only showed a tendency to reduce the residual variance. Despite inaccuracy in marker procedures, eFDMO from external markers provided a reliable determination for FE measurement. However, DMD estimated by internal markers did not improve prediction of FE, probably reflecting small variability. 相似文献
13.
Masahiko Gosho Kazushi Maruo Yasunori Sato 《Biometrical journal. Biometrische Zeitschrift》2014,56(6):991-1000
The accelerated failure time model is presented as an alternative to the proportional hazard model in the analysis of survival data. We investigate the effect of covariates omission in the case of applying a Weibull accelerated failure time model. In an uncensored setting, the asymptotic bias of the treatment effect is theoretically zero when important covariates are omitted; however, the asymptotic variance estimator of the treatment effect could be biased and then the size of the Wald test for the treatment effect is likely to exceed the nominal level. In some cases, the test size could be more than twice the nominal level. In a simulation study, in both censored and uncensored settings, Type I error for the test of the treatment effect was likely inflated when the prognostic covariates are omitted. This work remarks the careless use of the accelerated failure time model. We recommend the use of the robust sandwich variance estimator in order to avoid the inflation of the Type I error in the accelerated failure time model, although the robust variance is not commonly used in the survival data analyses. 相似文献
14.
Austin Burt Jacob G. Bundy Charles F. Baer 《Evolution; international journal of organic evolution》2016,70(10):2239-2246
A properly functioning organism must maintain metabolic homeostasis. Deleterious mutations degrade organismal function, presumably at least in part via effects on metabolic function. Here we present an initial investigation into the mutational structure of the Caenorhabditis elegans metabolome by means of a mutation accumulation experiment. We find that pool sizes of 29 metabolites vary greatly in their vulnerability to mutation, both in terms of the rate of accumulation of genetic variance (the mutational variance, VM) and the rate of change of the trait mean (the mutational bias, ΔM). Strikingly, some metabolites are much more vulnerable to mutation than any other trait previously studied in the same way. Although we cannot statistically assess the strength of mutational correlations between individual metabolites, principal component analysis provides strong evidence that some metabolite pools are genetically correlated, but also that there is substantial scope for independent evolution of different groups of metabolites. Averaged over mutation accumulation lines, PC3 is positively correlated with relative fitness, but a model in which metabolites are uncorrelated with fitness is nearly as good by Akaike's Information Criterion. 相似文献
15.
Publication bias and related types of small-study effects threaten the validity of systematic reviews. The existence of small-study effects has been demonstrated in empirical studies. Small-study effects are graphically diagnosed by inspection of the funnel plot. Though observed funnel plot asymmetry cannot be easily linked to a specific reason, tests based on funnel plot asymmetry have been proposed. Beyond a vast range of funnel plot tests, there exist several methods for adjusting treatment effect estimates for these biases. In this article, we consider the trim-and-fill method, the Copas selection model, and more recent regression-based approaches. The methods are exemplified using a meta-analysis from the literature and compared in a simulation study, based on binary response data. They are also applied to a large set of meta-analyses. Some fundamental differences between the approaches are discussed. An assumption common to the trim-and-fill method and the Copas selection model is that the small-study effect is caused by selection. The trim-and-fill method corresponds to an unknown implicit model generated by the symmetry assumption, whereas the Copas selection model is a parametric statistical model. However, it requires a sensitivity analysis. Regression-based approaches are easier to implement and not based on a specific selection model. Both simulations and applications suggest that in the presence of strong selection both the trim-and-fill method and the Copas selection model may not fully eliminate bias, while regression-based approaches seem to be a promising alternative. 相似文献
16.
Hans-Peter Piepho 《Biometrical journal. Biometrische Zeitschrift》2023,65(7):2200290
The coefficient of determination (R2) is a common measure of goodness of fit for linear models. Various proposals have been made for extension of this measure to generalized linear and mixed models. When the model has random effects or correlated residual effects, the observed responses are correlated. This paper proposes a new coefficient of determination for this setting that accounts for any such correlation. A key advantage of the proposed method is that it only requires the fit of the model under consideration, with no need to also fit a null model. Also, the approach entails a bias correction in the estimator assessing the variance explained by fixed effects. Three examples are used to illustrate new measure. A simulation shows that the proposed estimator of the new coefficient of determination has only minimal bias. 相似文献
17.
Veronique Etienne Erik C. Andersen José Miguel Ponciano Dustin Blanton Analucia Cadavid Joanna Joyner‐Matos Chikako Matsuba Brandon Tabman Charles F. Baer 《Evolution; international journal of organic evolution》2015,69(2):508-519
Understanding the genetic basis of susceptibility to pathogens is an important goal of medicine and of evolutionary biology. A key first step toward understanding the genetics and evolution of any phenotypic trait is characterizing the role of mutation. However, the rate at which mutation introduces genetic variance for pathogen susceptibility in any organism is essentially unknown. Here, we quantify the per‐generation input of genetic variance by mutation (VM) for susceptibility of Caenorhabditis elegans to the pathogenic bacterium Pseudomonas aeruginosa (defined as the median time of death, LT50). VM for LT50 is slightly less than VM for a variety of life‐history and morphological traits in this strain of C. elegans, but is well within the range of reported values in a variety of organisms. Mean LT50 did not change significantly over 250 generations of mutation accumulation. Comparison of VM to the standing genetic variance (VG) implies a strength of selection against new mutations of a few tenths of a percent. These results suggest that the substantial standing genetic variation for susceptibility of C. elegans to P. aeruginosa can be explained by polygenic mutation coupled with purifying selection. 相似文献
18.
19.
A. R. Sen 《Biometrical journal. Biometrische Zeitschrift》1979,21(2):131-137
A biased but simple and consistent estimator of the parameter ? has been obtained for the normal distribution N(?, a?2), ?>0 where a is a known constant. It is shown that the estimator is more efficient than the sample mean or any suitably chosen constant multiple of the sample standard deviation. It is also proved to be more efficient than the mimumum variance unbiased estimator among a typical class of unbiased estimators derived by RASUL KHAN (1968). 相似文献
20.
Michael N. Bruton Sheila E. Coutouvidis Jean Pote 《Environmental Biology of Fishes》1991,32(1-4):403-433
Synopsis A list of published references on the coelacanth Latimeria chalumnae is provided. All known publications in the scientific literature are included as well as popular articles and press reports that are considered to provide new information or interpretations. Marked trends are noticeable in the literature as different disciplines have been applied to research on the coelacanth over the past five decades. The bibliography lists a total of 823 publications including 490 papers in journals, 37 books, 3 theses, 45 chapters in books, 166 popular articles, 22 reports and 60 newspaper articles. Studies on taxonomy and morphology initially dominated the literature followed by reports on research in the fields of physiology, behaviour, breeding biology, ecology and conservation as frozen and eventually live specimens became available for study. The literature on the living coelacanth is predominantly in English, French, Japanese and German but references in 12 other languages were also traced. The dominant authors in the first decades of coelacanth research were the French scientists J. Millot and J. Anthony and the South African describer of the first and second coelacanths, J.L.B. Smith. In subsequent years French, British, American, South African, Japanese, Canadian and German authors, among others, have made significant contributions. 相似文献