1,553 research outputs found
Stochastic volatility models with possible extremal clustering
In this paper we consider a heavy-tailed stochastic volatility model,
, , where the volatility sequence
and the i.i.d. noise sequence are assumed independent, is
regularly varying with index , and the 's have moments of order
larger than . In the literature (see Ann. Appl. Probab. 8 (1998)
664-675, J. Appl. Probab. 38A (2001) 93-104, In Handbook of Financial Time
Series (2009) 355-364 Springer), it is typically assumed that
is a Gaussian stationary sequence and the 's are regularly varying with
some index (i.e., has lighter tails than the 's), or
that is i.i.d. centered Gaussian. In these cases, we see that the
sequence does not exhibit extremal clustering. In contrast to this
situation, under the conditions of this paper, both situations are possible;
may or may not have extremal clustering, depending on the clustering
behavior of the -sequence.Comment: Published in at http://dx.doi.org/10.3150/12-BEJ426 the Bernoulli
(http://isi.cbs.nl/bernoulli/) by the International Statistical
Institute/Bernoulli Society (http://isi.cbs.nl/BS/bshome.htm
Kliimamuutuste mõju leht- ja okaspuude peenjuurte omadustele
Väitekirja elektrooniline versioon ei sisalda publikatsiooneKliima, sademete, õhuniiskuse ja lämmastiku muutus mõjutab metsaökosüsteeme, eriti probleemsed on metsad turbamullal. Puude peenjuured jagatakse imi- ja transpordijuurteks, nende kõrge eripind ja -pikkus aitavad puul paremini ressursse omandada.
Töö eesmärk oli analüüsida leht- ja okaspuude peenjuure kohanemist kliima- ja mullaoludega. Selleks hinnati hübriidhaava, hõbekase ja männi juurte süsinikuvooge, juurte ehituste muutlikkust mulla niiskuse ja lämmastiku suurenedes. Kuivendatud metsades uuriti imijuurte ehituse muutumist sõltuvalt kaugusega kraavist, et teha kindlaks kase ja kuuse peenjuurte plastilisemad tunnused. Avaldatud uuringute põhjal analüüsiti ka puude juurte reageerimist mulla soojenemisele.
Uuringus ilmnesid mulla süsinikutasakaalu mõjutavad muutused. Kliimakambris vähenes kõrgenenud õhuniiskuse juures transpiratsioon. Männi fotosüntees kiirenes kõrgema õhuniiskuse juures. Maa-all vähendas kõrgem õhuniiskus oluliselt süsiniku vooge, suurenes juure eripind kasel, vähenes männil. Kuivendatud turbametsades kohanesid imijuured mulla seisundiga kasel ja kuusel sarnaselt, kaugusega kraavist suurenes keskmine juure eripind, koe tihedus vähenes.
Mulla soojenemine suurendas peenjuurte produktsiooni liigiti erinevalt, vähenes soojendav mõju massile. Mullatemperatuuri tõus suurendab juurte kasvukiirust. Soojemates oludes kasvatavad puud rohkem peenjuurte massi, morfoloogilist plastilisust mõjutatakse vähem. Otstarbekas oleks analüüsida peenjuurte tunnuseid koos mulla mikrobioomidega, eriti juurtega seotud ektomükoriisa seentega ja multifaktoriaalsetes keskkonnatingimustes.
Changes in global temperature, precipitation, air humidity, and N deposition pose challenges to forest ecosystems. Of particular concern are peatland forests. Fine roots (<2 mm in diameter) are divided into absorptive and transport roots. High specific root area and length mean higher resource acquisition.
The thesis aimed to analyze fine root acclimation of deciduous and coniferous trees to varying climate and soil. We evaluated fine root C fluxes and morphological variation of hybrid aspen, silver birch, and Scots pine in response to elevated humidity and soil inorganic N; the role of fine root functional groups in carbon exudation; and morphological variation of absorptive roots across the distance from the ditch in drained peatland forests dominated by birch and spruce to identify plasticity in fine root acclimation. Response of fine root biomass and morphology to soil warming at global scale was assessed in meta-analysis.
The study showed species-specific responses affecting soil carbon balance. In climate chambers, enhanced humidity reduced transpiration. Enhanced humidity caused growth in pine photosynthesis, decrease in belowground carbon fluxes. Specific root area increased in birch but decreased in pine. The absorptive root morphological responses were uniform in birch and spruce. With increased distance from the ditch, specific root area increased, tissue density decreased.
Soil warming increased fine root biomass differently for deciduous and coniferous species. The warming effects on fine root biomass decreased with greater warming magnitude. Rise in soil temperature stimulates root growth. Trees allocate more biomass to fine roots under warmer conditions, morphological plasticity is influenced less. More comprehensive analyses of fine root traits along with root-associated ectomycorrhizal fungi under multifactorial environmental conditions are needed.https://www.ester.ee/record=b555518
Evaluation of bred fish and seawater fish in terms of nutritional value, and heavy metals
In many parts of the world consumption of fish and seafood comprises a key proportion of man's diet and health. Despite of having many benefits, eating fish can be dangerous for instance the existence of nonorganic material, especially heavy metals, in some fish is dangerous. There are numerous fish breeding pools across the Lorestan province of Iran and the majority of the people living in these areas consume these kinds of fish, so, we were impelled to carry out a study to compare the nutrients and also heavy metals existent in freshwater fish and seawater fish available to the public across Khorramabad city of Iran. In this cross-sectional study, 9 samples of each five species of freshwater and sea water fish were purchased and their total protein, fat, omega 3, 6, and 9 fatty acids and also their heavy metals content including mercury, lead and cadmium of them were measured. There were no significant differences between mean protein content of the two types of fish. The amount of total fat and omega 3, 6 and 9 fatty acids of freshwater fish was higher than of seawater fish (P>0.001). The levels of cadmium in seawater fish was significantly higher than freshwater fish (P>0.001), and as for the level of mercury and lead, no significant difference was observed between the two types of freshwater fish and seawater fish. According to the results, we recommend that people can secure a part of their protein and unsaturated fatty acids need by consuming freshwater fish
Criteria of Validity and Reliability in Qualitative Research
Introduction: Today, with emerging qualitative methodology, questions about the validity and reliability in qualitative research findings have raised. According to different views about validity and reliability of qualitative research, this study aimed to determine the main criteria of validity and reliability in qualitative research from the view pointes of experts groups.
Method: In this cross-sectional descriptive research, internet scientific search was done in national and international valid databases including Irandoc, PubMed, CINAHL, ScienceDirect, and Thomson Reuters Web of Science using keywords about the validity and reliability of qualitative research. The articles focusing on the validity and reliability in qualitative research in the area of health and hygiene during the years after 2002 were extracted and assessed.
Results: A total of 25 research and review articles, and other resources related to reliability and validity of qualitative research since 2002 were extracted and assessed. There were variety of methods, and interpretive viewpoints about the validity and reliability of qualitative researches. Evaluation methods to gain credibility and reliability in qualitative researches depended on researchers' ontological, epistemological, and theoretical attitudes and insights.
Conclusion: Any researcher depending on his/her theoretical insights selects a separated method to assess the reliability of their qualitative researches. The reliability and validity in qualitative researches depend on asking principle questions about the nature of research, recognition of it, the logic governing it, and the trustworthiness of results.
Keywords: Trustworthiness, Qualitative research, Reliability, Validit
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