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Strong Assumptions (strong + assumption)
Selected AbstractsA New Look at Gender Effects in Participation and Occupation ChoiceLABOUR, Issue 3 2001Didier Soopramanien In this paper we evaluate the extent to which changes over time in women's labour market destinations are due to characteristics, on the one hand, and prices, on the other. Multinomial and nested logit methods are used to analyse US data for 1970 and 1990, and the results are compared. The latter method, which has not previously been employed in the present context, alleviates problems due to the strong assumption in simpler models of the independence of irrelevant alternatives, and provides much additional useful information. [source] Effect of current reproduction on apparent survival, breeding dispersal, and future reproduction in barn swallows assessed by multistate capture,recapture modelsJOURNAL OF ANIMAL ECOLOGY, Issue 3 2009Michael Schaub Summary 1Theoretical models predict a negative effect of current reproduction on breeding dispersal, survival and future reproduction, and many studies confirm these predictions. Yet, results of most previous studies may be difficult to interpret because the fate of the affected individuals cannot always be observed. Detection is almost always imperfect and some individuals emigrate from the study area, resulting in biased estimates of both survival and dispersal. 2Most studies bypass these problems with strong assumptions. We use a multistate capture,recapture model that does not require these assumptions. States are defined based on classes of reproductive success and on observed dispersal events within the study area. By accounting for imperfect detection within the study area, the model allows estimation of the effect of reproductive success on apparent survival, dispersal probabilities within the study area and the annual transition probabilities among classes of reproductive success. Based on an assumption about the estimate of real survival, the model allows the estimation of total dispersal that is not specific to a fixed study area. 3We applied this model to capture,recapture data of 2262 adult barn swallows (Hirundo rustica) sampled from 1997,2004 in eight local populations in Switzerland. 4We found that dispersal within the study area decreased with increasing reproductive success in both sexes, that reproductive success was not affected by preceding dispersal and that apparent survival of females but not of males increased with increasing reproductive success. Apparent survival of females with high reproductive success was identical to apparent survival of males suggesting that this estimate of apparent survival (0·48) was close to true survival. Total breeding dispersal was generally higher in females and it increased with decreasing reproductive success in both sexes. Current reproductive success depended on reproductive success in the preceding year suggesting that individual differences were of importance. 5Our study highlights that reproductive success was an important factor affecting breeding dispersal and population turnover. While unsuccessful males mainly remained in the local populations, many unsuccessful females left them. Population turnover of adult swallows was mainly due to unsuccessful females. [source] Estimation of immigration rate using integrated population modelsJOURNAL OF APPLIED ECOLOGY, Issue 2 2010Fitsum Abadi Summary 1.,The dynamics of many populations is strongly affected by immigrants. However, estimating and modelling immigration is a real challenge. In the past, several methods have been developed to estimate immigration rate but they either require strong assumptions or combine in a piecewise manner the results from separate analyses. In most methods the effects of covariates cannot be modelled formally. 2.,We developed a Bayesian integrated population model which combines capture,recapture data, population counts and information on reproductive success into a single model that estimates and models immigration rate, while directly assessing the impact of environmental covariates. 3.,We assessed parameter identifiability by comparing posterior distributions of immigration rates under varying priors, and illustrated the application of the model with long term demographic data of a little owl Athene noctua population from Southern Germany. We further assessed the impact of environmental covariates on immigration. 4.,The resulting posterior distributions were insensitive to different prior distributions and dominated by the observed data, indicating that the immigration rate was identifiable. Average yearly immigration into the little owl population was 0·293 (95% credible interval 0·183,0·418), which means that ca 0·3 female per resident female entered the population every year. Immigration rate tended to increase with increasing abundance of voles, the main prey of little owls. 5.Synthesis and applications. The means to estimate and model immigration is an important step towards a better understanding of the dynamics of geographically open populations. The demographic estimates obtained from the developed integrated population model facilitate population diagnoses and can be used to assess population viability. The structural flexibility of the model should constitute a useful tool for wildlife managers and conservation ecologists. [source] Efficiency in an Economy with Fixed CostsJOURNAL OF PUBLIC ECONOMIC THEORY, Issue 2 2001Andrea Dall'olio It is by now well known that in an economy with increasing returns, first-best efficiency may be impossible to attain through an equilibrium concept based on market prices, even if firms are regulated to follow marginal cost pricing. We examine the efficiency issue in a special but important class of economies in which the only source of nonconvexities is the presence of fixed costs. Even in this context, it is possible that none of the equilibria based on marginal cost pricing are efficient (unless additional, strong assumptions are made). We argue that available results on the existence of an efficient two-part tariff equilibrium rely on very strong assumptions, and we provide a positive result using a weak surplus condition. Our approach can also be used to establish the existence of an efficient marginal cost pricing equilibrium with endogenously chosen lump-sum taxes if the initial endowment is efficient in the economy without the production technology. [source] Analysis of longitudinal multiple-source binary data using generalized estimating equationsJOURNAL OF THE ROYAL STATISTICAL SOCIETY: SERIES C (APPLIED STATISTICS), Issue 1 2004Liam M. O'Brien Summary., We present a multivariate logistic regression model for the joint analysis of longitudinal multiple-source binary data. Longitudinal multiple-source binary data arise when repeated binary measurements are obtained from two or more sources, with each source providing a measure of the same underlying variable. Since the number of responses on each subject is relatively large, the empirical variance estimator performs poorly and cannot be relied on in this setting. Two methods for obtaining a parsimonious within-subject association structure are considered. An additional complication arises with estimation, since maximum likelihood estimation may not be feasible without making unrealistically strong assumptions about third- and higher order moments. To circumvent this, we propose the use of a generalized estimating equations approach. Finally, we present an analysis of multiple-informant data obtained longitudinally from a psychiatric interventional trial that motivated the model developed in the paper. [source] Efficient use of higher-lag autocorrelations for estimating autoregressive processesJOURNAL OF TIME SERIES ANALYSIS, Issue 3 2002LAURENCE BROZE The Yule,Walker estimator is commonly used in time-series analysis, as a simple way to estimate the coefficients of an autoregressive process. Under strong assumptions on the noise process, this estimator possesses the same asymptotic properties as the Gaussian maximum likelihood estimator. However, when the noise is a weak one, other estimators based on higher-order empirical autocorrelations can provide substantial efficiency gains. This is illustrated by means of a first-order autoregressive process with a Markov-switching white noise. We show how to optimally choose a linear combination of a set of estimators based on empirical autocorrelations. The asymptotic variance of the optimal estimator is derived. Empirical experiments based on simulations show that the new estimator performs well on the illustrative model. [source] A primer on classical test theory and item response theory for assessments in medical educationMEDICAL EDUCATION, Issue 1 2010André F De Champlain Context, A test score is a number which purportedly reflects a candidate's proficiency in some clearly defined knowledge or skill domain. A test theory model is necessary to help us better understand the relationship that exists between the observed (or actual) score on an examination and the underlying proficiency in the domain, which is generally unobserved. Common test theory models include classical test theory (CTT) and item response theory (IRT). The widespread use of IRT models over the past several decades attests to their importance in the development and analysis of assessments in medical education. Item response theory models are used for a host of purposes, including item analysis, test form assembly and equating. Although helpful in many circumstances, IRT models make fairly strong assumptions and are mathematically much more complex than CTT models. Consequently, there are instances in which it might be more appropriate to use CTT, especially when common assumptions of IRT cannot be readily met, or in more local settings, such as those that may characterise many medical school examinations. Objectives, The objective of this paper is to provide an overview of both CTT and IRT to the practitioner involved in the development and scoring of medical education assessments. Methods, The tenets of CCT and IRT are initially described. Then, main uses of both models in test development and psychometric activities are illustrated via several practical examples. Finally, general recommendations pertaining to the use of each model in practice are outlined. Discussion, Classical test theory and IRT are widely used to address measurement-related issues that arise from commonly used assessments in medical education, including multiple-choice examinations, objective structured clinical examinations, ward ratings and workplace evaluations. The present paper provides an introduction to these models and how they can be applied to answer common assessment questions. Medical Education 2010: 44: 109,117 [source] DEMAND POLICIES FOR LONG-RUN GROWTH: BEING KEYNESIAN BOTH IN THE SHORT AND IN THE LONG RUN?METROECONOMICA, Issue 1 2007Article first published online: 8 FEB 200, Marco Missaglia ABSTRACT The idea of demand-led growth is defended by neo-Kaleckians and neo-Keynesians using very specific assumptions. In their models the paradox of costs is always valid in the long run. The central message of this paper is that these specific and strong assumptions are not needed to defend the Kaleckian perspective of a demand-driven long-run growth. What is needed is simply a less demanding theory of flexible mark-ups in an open economy. The formal model developed in this paper shows that long-run growth may be demand driven even when the paradox of costs does not hold in the long run. [source] Statistical models of shape for the analysis of protein spots in two-dimensional electrophoresis gel imagesPROTEINS: STRUCTURE, FUNCTION AND BIOINFORMATICS, Issue 6 2003Mike Rogers Abstract In image analysis of two-dimensional electrophoresis gels, individual spots need to be identified and quantified. Two classes of algorithms are commonly applied to this task. Parametric methods rely on a model, making strong assumptions about spot appearance, but are often insufficiently flexible to adequately represent all spots that may be present in a gel. Nonparametric methods make no assumptions about spot appearance and consequently impose few constraints on spot detection, allowing more flexibility but reducing robustness when image data is complex. We describe a parametric representation of spot shape that is both general enough to represent unusual spots, and specific enough to introduce constraints on the interpretation of complex images. Our method uses a model of shape based on the statistics of an annotated training set. The model allows new spot shapes, belonging to the same statistical distribution as the training set, to be generated. To represent spot appearance we use the statistically derived shape convolved with a Gaussian kernel, simulating the diffusion process in spot formation. We show that the statistical model of spot appearance and shape is able to fit to image data more closely than the commonly used spot parameterizations based solely on Gaussian and diffusion models. We show that improvements in model fitting are gained without degrading the specificity of the representation. [source] Comparing methods for the multi-response design problemQUALITY AND RELIABILITY ENGINEERING INTERNATIONAL, Issue 5 2001John F. Kros Abstract One approach to solving multiple response engineering problems is to combine the individual responses into one unifying objective. In tility theory, several characteristics are used to compare and contrast multiple objective techniques. These are risk aversion, marginal rates of substitution, and the relationship of the responses in the combined function. Perhaps unknown to the user, multiple response techniques carry strong assumptions regarding these characteristics. This paper investigates four commonly-used multiple objective techniques and demonstrates that each method contains assumptions about these characteristics which are not intuitively evident to a user. Copyright © 2001 John Wiley & Sons, Ltd [source] Productivity growth and the returns from public investment in R&D in Australian broadacre agricultureAUSTRALIAN JOURNAL OF AGRICULTURAL & RESOURCE ECONOMICS, Issue 4 2007John Mullen Investment in R&D has long been regarded as an important source of productivity growth in Australian agriculture. Perhaps because research lags are long, current investment in R&D is monitored closely. Investment in R&D has been flat while productivity growth has remained strong, relative both to other sectors of the Australian economy and to the agricultural sectors of other countries. Such productivity growth, at a time when the decline in terms of trade facing Australian farmers has slowed, may have enhanced the competitiveness of Australian agriculture. The econometric results presented here suggest no evidence of a decline in the returns from research from the 15 to 40 per cent per annum range estimated by Mullen and Cox. In fact the marginal impact of research increases with research over the range of investment levels experienced from 1953 to 2000, a finding which lends support to the view that there is underinvestment in agricultural research. These results were obtained from econometric models which maintain strong assumptions about how investments in research and extension translate into changes in TFP. Hence some caution in interpreting the results is warranted. [source] Asymptotic Distribution of Score Statistics for Spatial Cluster Detection with Censored DataBIOMETRICS, Issue 4 2008Daniel Commenges SummaryCook, Gold, and Li (2007, Biometrics 63, 540,549) extended the Kulldorff (1997, Communications in Statistics 26, 1481,1496) scan statistic for spatial cluster detection to survival-type observations. Their approach was based on the score statistic and they proposed a permutation distribution for the maximum of score tests. The score statistic makes it possible to apply the scan statistic idea to models including explanatory variables. However, we show that the permutation distribution requires strong assumptions of independence between potential cluster and both censoring and explanatory variables. In contrast, we present an approach using the asymptotic distribution of the maximum of score statistics in a manner not requiring these assumptions. [source] | ||||||||||||||||