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Simple Explanation (simple + explanation)
Selected AbstractsA Simple Explanation of the Forecast Combination Puzzle,OXFORD BULLETIN OF ECONOMICS & STATISTICS, Issue 3 2009Jeremy Smith Abstract This article presents a formal explanation of the forecast combination puzzle, that simple combinations of point forecasts are repeatedly found to outperform sophisticated weighted combinations in empirical applications. The explanation lies in the effect of finite-sample error in estimating the combining weights. A small Monte Carlo study and a reappraisal of an empirical study by Stock and Watson [Federal Reserve Bank of Richmond Economic Quarterly (2003) Vol. 89/3, pp. 71,90] support this explanation. The Monte Carlo evidence, together with a large-sample approximation to the variance of the combining weight, also supports the popular recommendation to ignore forecast error covariances in estimating the weight. [source] Breath-holding and its breakpointEXPERIMENTAL PHYSIOLOGY, Issue 1 2006M. J. Parkes This article reviews the basic properties of breath-holding in humans and the possible causes of the breath at breakpoint. The simplest objective measure of breath-holding is its duration, but even this is highly variable. Breath-holding is a voluntary act, but normal subjects appear unable to breath-hold to unconsciousness. A powerful involuntary mechanism normally overrides voluntary breath-holding and causes the breath that defines the breakpoint. The occurrence of the breakpoint breath does not appear to be caused solely by a mechanism involving lung or chest shrinkage, partial pressures of blood gases or the carotid arterial chemoreceptors. This is despite the well-known properties of breath-hold duration being prolonged by large lung inflations, hyperoxia and hypocapnia and being shortened by the converse manoeuvres and by increased metabolic rate. Breath-holding has, however, two much less well-known but important properties. First, the central respiratory rhythm appears to continue throughout breath-holding. Humans cannot therefore stop their central respiratory rhythm voluntarily. Instead, they merely suppress expression of their central respiratory rhythm and voluntarily ,hold' the chest at a chosen volume, possibly assisted by some tonic diaphragm activity. Second, breath-hold duration is prolonged by bilateral paralysis of the phrenic or vagus nerves. Possibly the contribution to the breakpoint from stimulation of diaphragm muscle chemoreceptors is greater than has previously been considered. At present there is no simple explanation for the breakpoint that encompasses all these properties. [source] On the empirical association between poor health and low socioeconomic status at old ageHEALTH ECONOMICS, Issue 3 2002Christian Salas Abstract Epidemiologic studies using mortality rates as indicators of health fail to find any meaningful association between poor health and low socioeconomic status in older age-groups, whereas economic studies using self-assessed health consistently find a significant positive correlation, even after controlling for self-reporting errors. Such contradictory results have not been reported for working age individuals. A simple explanation might be that the elderly samples on which the epidemiologic and economic studies are based come from different populations. However, this paper shows that similar contradictory results are obtained even when the same samples are used, simply by switching between self-assessed health and mortality as health indicators. An alternative explanation is proposed, namely that these health indicators yield different results because they relate to different ranges of the latent health variable at old age. Copyright © 2002 John Wiley & Sons, Ltd. [source] Infrequent sporophyte production maintains a female-biased sex ratio in the unisexual clonal moss Hylocomium splendensJOURNAL OF ECOLOGY, Issue 5 2010Knut Rydgren Summary 1.,Sex ratios in unisexual bryophytes are most often female biased, whereas male-biased sex ratios predominate in unisexual seed plants. This ,bryophyte paradox', i.e. that sex ratios are biased in favour of the sex associated with the highest reproductive costs, has remained unexplained. 2.,Analysis of sex-ratio patterns via the influence of sex distribution on population growth rates (,) has not previously been carried out for bryophytes. We used this method to model how variation in sex ratio and sporophyte frequency influences , in the clonal bryophyte Hylocomium splendens. We obtained , by matrix modelling of synthetic experimental populations derived from demographic field data, using a linear two-sex model. 3.,In our set of experimental populations , varied between 1.13 and 1.27 in response to variation in sex ratio and sporophyte frequency, with the highest , obtained for the combination of a very low sporophyte frequency and a slightly female-biased sex ratio. 4.,Our results explain the female-biased sex ratio of H. splendens by the slightly lower survival of and production of vegetative offspring by males than by non-sporophytic females. 5.,Synthesis. According to our models, female dominance is the predicted outcome of low to moderate fertilization success and male performance intermediate between that of sporophytic and of non-sporophytic females. Our results therefore explain how a female-biased sex ratio can be maintained despite higher costs of reproduction in females than in males. In dioecious bryophytes, males and females must grow in close contact for fertilization to take place. Better performance of male ramets than of the female ramets they fertilize also explains how male clones can expand into female clones. A similar performance hierarchy of males and females may occur in unisexual clonal seed plants, but more efficient fertilization systems by pollination prevents the selective advantage of unfertilized females from being realized. This explains why vascular plant populations tend to be male biased. We hypothesise difference in fertilization distance range between sperm and pollen as a simple explanation why ramet level sex ratios are in general male dominated in clonal seed plants and female dominated in clonal bryophytes. [source] Rapid contrasting of ultrathin sections using microwave irradiation with heat dissipationJOURNAL OF MICROSCOPY, Issue 2 2001F. Hernández-Chavarría The use of microwave irradiation (MWI) to accelerate fixation, dehydration and contrasting (staining) for electron microscopy has been applied to the development of rapid methods to process biological samples in electron microscopy. A simple explanation is that the reduced time in those procedures is due to heating. In this paper we propose a contrasting method for thin sections that avoids the thermal effects of MWI. Grids with thin sections of mouse kidney, the dinoflagellate Alexandrium monilatum, spermatophores of the fly Archicepsis diversiformis, the bacteria Acinetobacter calcoaceticum and Enterobacter cloacae were placed into Beem capsules and stained with uranyl acetate and lead citrate, while immersed in an ice-water bath, and irradiated for periods ranging from 30 s to 2 min. After each contrasting procedure, the Beem capsule was filled with distilled water to wash the grids under MWI with the same irradiation time as used to contrast. Good results were obtained on irradiating for 1 min and the temperature of the Beem capsule was maintained around 5 °C. [source] Phylogeography of sexual Heteronotia binoei (Gekkonidae) in the Australian arid zone: climatic cycling and repetitive hybridizationMOLECULAR ECOLOGY, Issue 9 2005JARED L. STRASBURG Abstract The biota of much of continental Australia have evolved within the context of gradual aridification of the region over several million years, and more recently of climatic cycling between relatively dry and humid conditions. We performed a phylogeographical study of three sexual chromosome races of the Heteronotia binoei complex of geckos found throughout the Australian arid zone. Two of these three races were involved in two separate hybridization events leading to parthenogenetic lineages (also H. binoei), and the third is widespread and broadly sympatric with the parthenogens. Based on our analyses, the three sexual races diversified approximately 6 million years ago in eastern Australia, during a period of aridification, then each moved west through northern, southern, and central dispersal corridors to occupy their current ranges. In each case, the timing of major phylogeographical inferences corresponds to inferred palaeoclimatic changes in continental Australia. This scenario provides a simple explanation for diversification, secondary contact, and hybridization between the races. However, data presented elsewhere indicate that formation of the parthenogens was considerably more recent than the westward expansion of the hybridizing races, and that multiple hybridization events were geographically and temporally distinct. We suggest that cyclical climate changes may have led to regional range changes that facilitated hybridization between the races, which are not currently known to be in sympatry. [source] Natural selection for salt tolerance quantitative trait loci (QTLs) in wild sunflower hybrids: Implications for the origin of Helianthus paradoxus, a diploid hybrid speciesMOLECULAR ECOLOGY, Issue 5 2003C. Lexer Abstract For a new diploid or homoploid hybrid species to become established, it must diverge ecologically from parental genotypes. Otherwise the hybrid neospecies will be overcome by gene flow or competition. We initiated a series of experiments designed to understand how the homoploid hybrid species, Helianthus paradoxus, was able to colonize salt marsh habitats, when both of its parental species (H. annuus×H. petiolaris) are salt sensitive. Here, we report on the results of a quantitative trait locus (QTL) analysis of mineral ion uptake traits and survivorship in 172 BC2 hybrids between H. annuus and H. petiolaris that were planted in H. paradoxus salt marsh habitat in New Mexico. A total of 14 QTLs were detected for mineral ion uptake traits and three for survivorship. Several mineral ion QTLs mapped to the same position as the survivorship QTLs, confirming previous studies, which indicated that salt tolerance in Helianthus is achieved through increased Ca uptake, coupled with greater exclusion of Na and related mineral ions. Of greater general significance was the observation that QTLs with effects in opposing directions were found for survivorship and for all mineral ion uptake traits with more than one detected QTL. This genetic architecture provides an ideal substrate for rapid ecological divergence in hybrid neospecies and offers a simple explanation for the colonization of salt marsh habitats by H. paradoxus. Finally, selection coefficients of +0.126, ,0.084 and ,0.094 for the three survivorship QTLs, respectively, are sufficiently large to account for establishment of new, homoploid hybrid species. [source] Perspektiven für mehr Jobs in DeutschlandPERSPEKTIVEN DER WIRTSCHAFTSPOLITIK, Issue 4 2001Klaus F. Zimmermann There is no simple solution to the German unemployment dilemma. Neither too high wages, nor unionized central bargaining, nor the evidence of the effects of the welfare state deliver a simple explanation. The East German experience confirms this view. A solution needs to combine flexibility with economic growth. More flexible product markets could help to generate the higher demand for unskilled labour that is so badly needed. [source] Forecast Dispersion and the Cross Section of Expected ReturnsTHE JOURNAL OF FINANCE, Issue 5 2004TIMOTHY C. JOHNSON ABSTRACT Recent work by Diether, Malloy, and Scherbina (2002) has established a negative relationship between stock returns and the dispersion of analysts' earnings forecasts. I offer a simple explanation for this phenomenon based on the interpretation of dispersion as a proxy for unpriced information risk arising when asset values are unobservable. The relationship then follows from a general options-pricing result: For a levered firm, expected returns should always decrease with the level of idiosyncratic asset risk. This story is formalized with a straightforward model. Reasonable parameter values produce large effects, and the theory's main empirical prediction is supported in cross-sectional tests. [source] Reloading the revolver , male fitness as a simple explanation for complex reward partitioning in Nasa macrothyrsa (Loasaceae, Cornales)BIOLOGICAL JOURNAL OF THE LINNEAN SOCIETY, Issue 1 2010MAXIMILIAN WEIGEND Reward partitioning and replenishment and specific mechanisms for pollen presentation are all geared towards the maximization of the number of effective pollinator visits to individual flowers. An extreme case of an apparently highly specialized plant,pollinator interaction with thigmonastic pollen presentation has been described for the morphologically complex tilt-revolver flowers of Caiophora arechavaletae (Loasaceae) pollinated by oligolectic Bicolletes pampeana (Colletidae, Hymenoptera). We studied the floral biology of Nasa macrothyrsa (Loasaceae) in the field and in the glasshouse, which has very similar floral morphology, but is pollinated by polylectic Neoxylocopa bees (Apidae, Hymenoptera). We investigated the presence of thigmonastic anther presentation, visitor behaviour (pollinators and nectar robbers), co-ordination of pollinator visits with flower behaviour and the presence of nectar replenishment. The aim of this study was to understand whether complex flower morphology and behaviour can be explained by a specialized pollination syndrome, or whether alternative explanations can be offered. The results showed that Nasa macrothyrsa has thigmonastic pollen presentation, i.e. new pollen is rapidly (<< 10 min) presented after a pollinator visit. Nectar secretion is independent of removal and averages 7,14 µL h,1. The complex flowers, however, fail to exclude either native (hummingbirds) or introduced (honeybees) nectar robbers, nor does polylectic Neoxylocopa actively collect the pollen presented. The findings do not support a causal link between complex flower morphology and functionality in Loasaceae and a highly specialized pollination. Rapid pollen presentation is best explained by the pollen presentation theory: the large proportion of pollinators coming shortly after a previous visit find little nectar and are more likely to move on to a different plant. The rapid presentation of pollen ensures that all these valuable ,hungry pollinators' are dusted with small pollen loads, thus increasing the male fitness of the plant by increasing the likelihood of siring outcrossed offspring. © 2010 The Linnean Society of London, Biological Journal of the Linnean Society, 2010, 100, 124,131. [source] Dualistic actions of cromakalim and new potent 2H -1,4-benzoxazine derivatives on the native skeletal muscle KATP channelBRITISH JOURNAL OF PHARMACOLOGY, Issue 2 2003Domenico Tricarico New 2H -1,4-benzoxazine derivatives were synthesized and tested for their agonist properties on the ATP-sensitive K+ channels (KATP) of native rat skeletal muscle fibres by using the patch-clamp technique. The novel modifications involved the introduction at position 2 of the benzoxazine ring of alkyl substituents such as methyl (,CH3), ethyl (,C2H5) or propyl (,C3H7) groups, while maintaining pharmacophore groups critical for conferring agonist properties. The effects of these molecules were compared with those of cromakalim in the presence or absence of internal ATP (10,4M). In the presence of internal ATP, all the compounds increased the macropatch KATP currents. The order of potency of the molecules as agonists was ,C3H7 (DE50=1.63 × 10,8M) >,C2H5 (DE50=1.11 × 10,7M)>,CH3 (DE50=2.81 × 10,7M)>cromak-slim (DE50= 1.42 × 10,5M). Bell-shaped dose,response curves were observed for these compounds and cromakalim indicating a downturn in response when a certain dose was exceeded. In contrast, in the absence of internal ATP, all molecules including cromakalim inhibited the KATP currents. The order of increasing potency as antagonists was cromakalim (IC50=1.15 × 10,8M),CH3 (IC50=2.6 × 10,8M)>,C2H5 (IC50=4.4 × 10,8M)>,C3H7 (IC50=1.68 × 10,7M) derivatives. These results suggest that the newly synthesized molecules and cromakalim act on muscle KATP channel by binding on two receptor sites that have opposite actions. Alternatively, a more simple explanation is to consider the existence of a single site for potassium channel openers regulated by ATP which favours the transduction of the channel opening. The alkyl chains at position 2 of the 2H -1,4-benzoxazine nucleus is pivotal in determining the potency of benzoxazine derivatives as agonists or antagonists. British Journal of Pharmacology (2003) 139, 255,262. doi:10.1038/sj.bjp.0705233 [source] Relationship of food frequencies as reported by parents to overweight and obesity at 5 yearsACTA PAEDIATRICA, Issue 1 2009Karina Huus Abstract Aim: To investigate if food frequencies are related to overweight/obesity in 5-year-old children. Methods: During 1997,1999, 21 700 infants were invited to participate in ABIS (All Babies in Southeast Sweden), a prospective, cohort study. Participants were followed from birth (n = 16 058) to 5 years (n = 7356). Food frequencies reported by parents at 2.5 and 5 years were studied in the relation to overweight/obesity at 5 years using multiple logistic regressions. A p-value < 0.01 was considered statistically significant. Results: At 2.5 years frequencies of intake of cheese were positively associated with overweight/obesity at 5 years while porridge, fried potatoes/french fries and cream/crème fraiche showed a negative association. When adjusting for known risk factors, porridge and fried potatoes/french fries remained negatively associated with overweight/obesity. At 5 years, chocolate and lemonade were positively associated with overweight/obesity whereas cream/crème fraiche, pastries and candy were negatively associated. Candy remained negatively associated to overweight/obesity after adjustment for potential confounders. Conclusion: Food frequencies do not offer any simple explanation for overweight/obesity. Porridge at 2.5 years may protect against overweight/obesity at 5 years, while lemonade may contribute to overweight. Our finding that fried potatoes/french fries may protect against overweight/obesity is unexpected and must be interpreted with caution. These findings should be confirmed by prospective studies using objective recordings. [source] Optic nerve blood flow in glaucomaCLINICAL AND EXPERIMENTAL OPTOMETRY, Issue 3 2000Renuka Bathija FRACO FRACS Background: Glaucomatous optic neuropathy often occurs in the absence of elevated intraocular pressure and, conversely, elevated intraocular pressure may occur without associated damage of the optic nerve. These findings challenge the simple explanation of intraocular pressure being the sole cause of neural loss and have led to theories of ischaemic causes of the morbidity. This paper reviews the vascular anatomy of the optic disc, the factors that control its blood flow and the existing techniques for measurement of the blood flow. It also briefly discusses the possible role of apoptosis in glaucomatous visual loss. Method: Literature review. Conclusions: The posterior ciliary artery circulation is the main source of the blood supply to the optic nerve head with additional lesser supply via the central retinal artery and the choroidal circulation. There is considerable individual variation in the distribution of this circulation and complex regulatory systems govern its function. It is likely that microcirculatory changes in the vascular supply of the optic disc play a role in glaucoma, either as the primary abnormality or as a co-factor that increases susceptibility to damage from increased intraocular pressure through impaired auto-regulation. Clinical trials are currently in progress for the treatment of glaucoma with systemically administered agents that are antagonists of the receptors that mediate glutamine toxicity, a factor in the process of apoptosis. [source] | ||||||||||||||||