255 resultados para Muestreo (Estadística )
Resumo:
Globularia alypum s un arbust termfil tpic de la regi oest del Mediterrani. Aquest nanofanerfit t un patr de floraci molt ampli. Les poblacions primerenques floreixen des del Juliol fins al Desembre i les poblacions tardanes des del Gener fins a lAbril. Lobjectiu del projecte s esbrinar quins factors climtics determinen la fenologia de la floraci de lespcie a partir de mostres dherbari i conixer labast geogrfic dambds patrons de floraci a la pennsula Ibrica i Frana. Tamb lestudi daltres caracterstiques fenolgiques en relaci amb el clima: els braquiblasts i les espigues de captols. Els plecs dherbari han estat georeferenciats per ser utilitzats en un entorn SIG junt amb les dades del clima i sha realitzat una anlisi estadística. El resultats mostren que les poblacions primerenques creixen majoritriament en zones costaneres mentre que les tardanes ho fan en zones continentals. Les poblacions primerenques creixen en rees on les temperatures (mitjana, mnima, mxima i mnima del mes ms fred) sn ms altes que les tardanes per lefecte temperador del mar. Al analitzar totes les poblacions, el nombre de mesos des de la transici floral es correlaciona negativament amb la temperatura mnima mensual i la temperatura mitjana mensual en tots els mesos, mentre que amb la temperatura mxima mensual entre Octubre i Abril. Al estudiar sols les poblacions tardanes les correlacions sn similars, per no en les primerenques on no sen troba cap. Les correlacions entre el nombre de mesos des de la transici floral i les variables climtiques anuals sn iguals, a ms de trobar-se una correlaci negativa amb la temperatura mnima del mes ms fred i lndex trmic, i positiva amb lndex de continentalitat. No sha trobat cap diferncia en presncia de braquiblasts segons la poblaci, per s en les espigues de captols, essent ms freqents en les poblacions primerenques. Es suggereix que la temperatura s un factor ms determinant per a les poblacions tardanes, mentre que es desconeix si s la precipitaci en les primerenques. Tampoc es t coneixement sobre si les diferncies entre els dos patrons sn gentiques o fenotpiques i quin mecanisme possibilita el desenvolupament de braquiblasts i espigues de captols.
Resumo:
Amb la finalitat davaluar els lpids alquilats com a proxies de precipitaci i temperatura a la Pennsula Ibrica, shan analitzat 23 mostres de sls de diferents punts del territori espanyol. Lestudi daquests biomarcadors cuticulars sha fet a travs de la identificaci i quantificaci del seu contingut a les mostres, el clcul de diversos parmetres com lndex de preferncia de carboni (CPI), la longitud mitjana de cadena (ACL) o la prdua de matria orgnica per ignici (LOI), i la recerca de correlacions estadstiques entre les variables climtiques dels punts de mostreig i els resultats obtinguts. Aquests conclouen amb el fet de que els lpids alquilats sn tils com a biomarcadors cuticulars de plantes superiors i tenen un potencial significatiu com a proxies de precipitaci a la Pennsula Ibrica.
Resumo:
A method to estimate an extreme quantile that requires no distributional assumptions is presented. The approach is based on transformed kernel estimation of the cumulative distribution function (cdf). The proposed method consists of a double transformation kernel estimation. We derive optimal bandwidth selection methods that have a direct expression for the smoothing parameter. The bandwidth can accommodate to the given quantile level. The procedure is useful for large data sets and improves quantile estimation compared to other methods in heavy tailed distributions. Implementation is straightforward and R programs are available.
Resumo:
Conixer la qualitat de vida (QV) mitjanant el qestionari Minnesota Living With Heart Failure Questionnaire (MLWHFQ) en una poblaci afecta dinsuficincia cardaca atesa al nivell datenci primria mitjanant un estudi descriptiu transversal i observacional. La major part dels pacients sn dones dedat avanada amb disfunci diastlica, detiologia hipertensiva. Laplicaci del MLWHFQ ha presentat puntuacions baixes. Sha trobat significaci estadística amb la classe funcional i el nombre dingressos en lltim any, en malalts amb malaltia pulmonar obstructiva crnica i insuficincia renal crnica. No sha trobat correlaci significativa amb la fracci dejecci, el tractament, ni amb la causa de la insuficincia cardaca.
Resumo:
All of the imputation techniques usually applied for replacing values below thedetection limit in compositional data sets have adverse effects on the variability. In thiswork we propose a modification of the EM algorithm that is applied using the additivelog-ratio transformation. This new strategy is applied to a compositional data set and theresults are compared with the usual imputation techniques
Resumo:
In the eighties, John Aitchison (1986) developed a new methodological approach for the statistical analysis of compositional data. This new methodology was implemented in Basic routines grouped under the name CODA and later NEWCODA inMatlab (Aitchison, 1997). After that, several other authors have published extensions to this methodology: Marn-Fernndez and others (2000), Barcel-Vidal and others (2001), Pawlowsky-Glahn and Egozcue (2001, 2002) and Egozcue and others (2003). (...)
Resumo:
The log-ratio methodology makes available powerful tools for analyzing compositionaldata. Nevertheless, the use of this methodology is only possible for those data setswithout null values. Consequently, in those data sets where the zeros are present, aprevious treatment becomes necessary. Last advances in the treatment of compositionalzeros have been centered especially in the zeros of structural nature and in the roundedzeros. These tools do not contemplate the particular case of count compositional datasets with null values. In this work we deal with \count zeros" and we introduce atreatment based on a mixed Bayesian-multiplicative estimation. We use the Dirichletprobability distribution as a prior and we estimate the posterior probabilities. Then weapply a multiplicative modication for the non-zero values. We present a case studywhere this new methodology is applied.Key words: count data, multiplicative replacement, composition, log-ratio analysis
Resumo:
In a seminal paper, Aitchison and Lauder (1985) introduced classical kernel densityestimation techniques in the context of compositional data analysis. Indeed, they gavetwo options for the choice of the kernel to be used in the kernel estimator. One ofthese kernels is based on the use the alr transformation on the simplex SD jointly withthe normal distribution on RD-1. However, these authors themselves recognized thatthis method has some deficiencies. A method for overcoming these dificulties based onrecent developments for compositional data analysis and multivariate kernel estimationtheory, combining the ilr transformation with the use of the normal density with a fullbandwidth matrix, was recently proposed in Martn-Fernndez, Chacn and Mateu-Figueras (2006). Here we present an extensive simulation study that compares bothmethods in practice, thus exploring the finite-sample behaviour of both estimators
Resumo:
The quantitative estimation of Sea Surface Temperatures from fossils assemblages is afundamental issue in palaeoclimatic and paleooceanographic investigations. TheModern Analogue Technique, a widely adopted method based on direct comparison offossil assemblages with modern coretop samples, was revised with the aim ofconforming it to compositional data analysis. The new CODAMAT method wasdeveloped by adopting the Aitchison metric as distance measure. Modern coretopdatasets are characterised by a large amount of zeros. The zero replacement was carriedout by adopting a Bayesian approach to the zero replacement, based on a posteriorestimation of the parameter of the multinomial distribution. The number of modernanalogues from which reconstructing the SST was determined by means of a multipleapproach by considering the Proxies correlation matrix, Standardized Residual Sum ofSquares and Mean Squared Distance. This new CODAMAT method was applied to theplanktonic foraminiferal assemblages of a core recovered in the Tyrrhenian Sea.Kew words: Modern analogues, Aitchison distance, Proxies correlation matrix,Standardized Residual Sum of Squares
Resumo:
s conegut que arreu del mn hi ha moltes poblacions allades que encara viuen a partir de lagricultura i la ramaderia de subsistncia i que utilitzen tots aquells recursos del seu entorn per tal de cobrir les serves necessitats bsiques. Aquest s acostuma a ser insostenible, fent que lentorn tendeixi a degradar-se cada cop ms. El projecte representa una pea bsica en la inserci del desenvolupament sostenible per a moltes daquestes poblacions, com s el cas de les comunes el riu Lules (Tucumn, Argentina). Basat en entrevistes i enquestes als pobladors, es fa una anlisi estadística per determinar ls i la percepci de canvi que tenen els habitants, i a la vegada, es relaciona amb altres variables com el nivell socioeconmic i ledat, entre daltres. Aquesta anlisi juntament amb lobservaci directa, permet fer una diagnosi de lrea i, conseqentment, una llista de propostes (i desenvolupament de les ms urgents) per tal de crear un pla de gesti de la zona i poder assegurar el seu desenvolupament sostenible
Resumo:
A graphical processing unit (GPU) is a hardware device normally used to manipulate computer memory for the display of images. GPU computing is the practice of using a GPU device for scientific or general purpose computations that are not necessarily related to the display of images. Many problems in econometrics have a structure that allows for successful use of GPU computing. We explore two examples. The first is simple: repeated evaluation of a likelihood function at different parameter values. The second is a more complicated estimator that involves simulation and nonparametric fitting. We find speedups from 1.5 up to 55.4 times, compared to computations done on a single CPU core. These speedups can be obtained with very little expense, energy consumption, and time dedicated to system maintenance, compared to equivalent performance solutions using CPUs. Code for the examples is provided.
Resumo:
In Catalonia, according to the nitrate directive (91/676/EU), nine areas have been declared as vulnerable to nitrate pollution from agricultural sources (Decret 283/1998 and Decret 479/2004). Five of these areas have been studied coupling hydro chemical data with a multi-isotopic approach (Vitria et al. 2005, Otero et al. 2007, Puig et al. 2007), in an ongoing research project looking for an integrated application of classical hydrochemistry data, with a comprehensive isotopic characterisation (15N and 18O of dissolved nitrate, 34S and 18O of dissolved sulphate, 13C of dissolved inorganic carbon, and D and 18O of water). Within this general frame, the contribution presented explores compositional ways of: (i) distinguish agrochemicals and manure N pollution, (ii) quantify natural attenuation of nitrate (denitrification), and identify possible controlling factors.To achieve this two-fold goal, the following techniques have been used. Separate biplots of each suite of data show that each studied region has a distinct 34S and pH signatures, but they are homogeneous with regard to NO3- related variables. Also, the geochemical variables were projected onto the compositional directions associated with the possible denitrification reactions in each region. The resulting balances can be plot together with some isotopes, to assess their likelihood of occurrence
Resumo:
One of the tantalising remaining problems in compositional data analysis lies in how to deal with data sets in which there are components which are essential zeros. By anessential zero we mean a component which is truly zero, not something recorded as zero simply because the experimental design or the measuring instrument has not been sufficiently sensitive to detect a trace of the part. Such essential zeros occur inmany compositional situations, such as household budget patterns, time budgets,palaeontological zonation studies, ecological abundance studies. Devices such as nonzero replacement and amalgamation are almost invariably ad hoc and unsuccessful insuch situations. From consideration of such examples it seems sensible to build up amodel in two stages, the first determining where the zeros will occur and the secondhow the unit available is distributed among the non-zero parts. In this paper we suggest two such models, an independent binomial conditional logistic normal model and a hierarchical dependent binomial conditional logistic normal model. The compositional data in such modelling consist of an incidence matrix and a conditional compositional matrix. Interesting statistical problems arise, such as the question of estimability of parameters, the nature of the computational process for the estimation of both the incidence and compositional parameters caused by the complexity of the subcompositional structure, the formation of meaningful hypotheses, and the devising of suitable testing methodology within a lattice of such essential zero-compositional hypotheses. The methodology is illustrated by application to both simulated and real compositional data
Resumo:
The application of Discriminant function analysis (DFA) is not a new idea in the studyof tephrochrology. In this paper, DFA is applied to compositional datasets of twodifferent types of tephras from Mountain Ruapehu in New Zealand and MountainRainier in USA. The canonical variables from the analysis are further investigated witha statistical methodology of change-point problems in order to gain a betterunderstanding of the change in compositional pattern over time. Finally, a special caseof segmented regression has been proposed to model both the time of change and thechange in pattern. This model can be used to estimate the age for the unknown tephrasusing Bayesian statistical calibration
Resumo:
Several eco-toxicological studies have shown that insectivorous mammals, due to theirfeeding habits, easily accumulate high amounts of pollutants in relation to other mammal species. To assess the bio-accumulation levels of toxic metals and their inuenceon essential metals, we quantified the concentration of 19 elements (Ca, K, Fe, B, P,S, Na, Al, Zn, Ba, Rb, Sr, Cu, Mn, Hg, Cd, Mo, Cr and Pb) in bones of 105 greaterwhite-toothed shrews (Crocidura russula) from a polluted (Ebro Delta) and a control(Medas Islands) area. Since chemical contents of a bio-indicator are mainly compositional data, conventional statistical analyses currently used in eco-toxicology can givemisleading results. Therefore, to improve the interpretation of the data obtained, weused statistical techniques for compositional data analysis to define groups of metalsand to evaluate the relationships between them, from an inter-population viewpoint.Hypothesis testing on the adequate balance-coordinates allow us to confirm intuitionbased hypothesis and some previous results. The main statistical goal was to test equalmeans of balance-coordinates for the two defined populations. After checking normality,one-way ANOVA or Mann-Whitney tests were carried out for the inter-group balances
