27,968 research outputs found
Publication Bias Against Null Results
No full textStudies suggest a bias against the publication of null (p > .05) results. Instead of significance, we advocate reporting effect sizes and confidence intervals, and using replication studies. If statistical tests are used, power tests should accompany them.publication, bias, null results
Linear Operator Inequality and Null Controllability with Vanishing Energy for Unbounded Control Systems
No full textWe consider a linear boundary or point control system on a Hilbert space which is null controllable at some time . To every initial state we associate the minimal ``energy'' needed to transfer to in a time (``energy'' of a control being the square of its norm). Clearly, it decreases with the control time . We shall prove that, under suitable spectral properties of the linear system operator, the minimal energy converges to for $ T\to+\infty
The appearance, motion, and disappearance of three-dimensional magnetic null points
No full textN.A.M. acknowledges support from NASA grants NNX11AB61G, NNX12AB25G, and NNX15AF43G; NASA contract NNM07AB07C; and NSF SHINE grants AGS-1156076 and AGS-1358342 to SAO. C.E.P. acknowledges support from the St Andrews 2013 STFC Consolidated grant.While theoretical models and simulations of magnetic reconnection often assume symmetry such that the magnetic null point when present is co-located with a flow stagnation point, the introduction of asymmetry typically leads to non-ideal flows across the null point. To understand this behavior, we present exact expressions for the motion of three-dimensional linear null points. The most general expression shows that linear null points move in the direction along which the magnetic field and its time derivative are antiparallel. Null point motion in resistive magnetohydrodynamics results from advection by the bulk plasma flow and resistive diffusion of the magnetic field, which allows non-ideal flows across topological boundaries. Null point motion is described intrinsically by parameters evaluated locally; however, global dynamics help set the local conditions at the null point. During a bifurcation of a degenerate null point into a null-null pair or the reverse, the instantaneous velocity of separation or convergence of the null-null pair will typically be infinite along the null space of the Jacobian matrix of the magnetic field, but with finite components in the directions orthogonal to the null space. Not all bifurcating null-null pairs are connected by a separator. Furthermore, except under special circumstances, there will not exist a straight line separator connecting a bifurcating null-null pair. The motion of separators cannot be described using solely local parameters because the identification of a particular field line as a separator may change as a result of non-ideal behavior elsewhere along the field line.Peer reviewe
Interpreting null findings from trials of alcohol brief interventions
No full textThe effectiveness of alcohol brief intervention (ABI) has been established by a succession of meta-analyses but, because the effects of ABI are small, null findings from randomized controlled trials are often reported and can sometimes lead to skepticism regarding the benefits of ABI in routine practice. This article first explains why null findings are likely to occur under null hypothesis significance testing (NHST) due to the phenomenon known as ‘the dance of the p-values’. A number of misconceptions about null findings are then described, using as an example the way in which the results of the primary care arm of a recent cluster randomized trial of ABI in England (the SIPS project) have been misunderstood. These misinterpretations include the fallacy of ‘proving the null hypothesis’ that lack of a significant difference between the means of sample groups can be taken as evidence of no difference between their population means, and the possible effects of this and related misunderstandings of the SIPS findings are examined. The mistaken inference that reductions in alcohol consumption seen in control groups from baseline to follow-up are evidence of real effects of control group procedures is then discussed and other possible reasons for such reductions, including regression to the mean, research participation effects, historical trends, and assessment reactivity, are described. From the standpoint of scientific progress, the chief problem about null findings under the conventional NHST approach is that it is not possible to distinguish ‘evidence of absence’ from ‘absence of evidence’. By contrast, under a Bayesian approach, such a distinction is possible and it is explained how this approach could classify ABIs in particular settings or among particular populations as either truly ineffective or as of unknown effectiveness, thus accelerating progress in the field of ABI research
Influence of the MDM2 single nucleotide polymorphism SNP309 on tumour development in BRCA1 mutation carriers
No full textBackgroundThe MDM2 gene encodes a negative regulator of the p53 tumour suppressor protein. A single nucleotide polymorphism (SNP) in the MDM2 promoter (a T to G exchange at nucleotide 309) has been reported to produce accelerated tumour formation in individuals with inherited p53 mutations. We have investigated the effect of the MDM2 SNP309 on clinical outcome in a cohort of patients with germline mutations of BRCA1.MethodsGenomic DNA was obtained for 102 healthy controls and 116 patients with established pathogenic mutations of BRCA1 and Pyrosequencing technology™ was used to determine the genotype at the MDM2 SNP309 locus.ResultsThe polymorphism was present in 52.9% of the controls (G/T in 37.3% and G/G in 15.6%) and 58.6% of the BRCA1 mutation carriers (47.4% G/T and 11.2% G/G). Incidence of malignancy in female BRCA1 carriers was not significantly higher in SNP309 carriers than in wildtype (T/T) individuals (72.7% vs. 75.6%, p = 1.00). Mean age of diagnosis of first breast cancer was 41.2 years in the SNP309 G/G genotype carriers, 38.6 years in those with the SNP309 G/T genotype and 39.0 years in wildtype subjects (p = 0.80).ConclusionWe found no evidence that the MDM2 SNP309 accelerates tumour development in carriers of known pathogenic germline mutations of BRCA1
MHD wave propagation in the neighbourhood of a two-dimensional null point
No full textThe nature of fast magnetoacoustic and Alfvén waves is investigated in a zero β plasma. This gives an indication of wave propagation in the low β solar corona. It is found that for a two-dimensional null point, the fast wave is attracted to that point and the front of the wave slows down as it approaches the null point, causing the current density to accumulate there and rise rapidly. Ohmic dissipation will extract the energy in the wave at this point. This illustrates that null points play an important role in the rapid dissipation of fast magnetoacoustic waves and suggests the location where wave heating will occur in the corona. The Alfvén wave behaves in a different manner in that the wave energy is dissipated along the separatrices. For Alfvén waves that are decoupled from fast waves, the value of the plasma β is unimportant. However, the phenomenon of dissipating the majority of the wave energy at a specific place is a feature of both wave types
Nullforms, polarization and tensorpowers
No full textPart I: Singular Spaces of the Nullcone:
Given a complex reductive group G and a complex representation V , one of the main
goals of invariant theory is to describe - in terms of generators and relations - the
ring of invariant polynomial functions, denoted by O(V )G. However, for most pairs
G and V , finding explicitly all generators of O(V )G is very difficult. An important
step in this search is to find homogeneous invariants whose zero set is the nullcone
Nv ⊂ V , i.e. the zero set of all homogeneous non-constant invariant functions on
V . Such invariants are strongly related to O(V )G as Hilbert proved the following
result: If f1, . . . , fr are homogeneous invariants whose zero set is equal to Nv then
O(V )G is a finitely generated module over the subalgebra C[f1, . . . , fr].
Given some invariants fi " O(V )G as above one can apply the so called polarization
process to obtain a set of functions lying in O(V ⊕k)G. Our main interest in
this work is to analyze whether the set of functions obtained in this manner defines
the nullcone NV !k . Due to an observation of Kraft and Wallach, this is equivalent
to the question whether for every linear subspace H ⊂ Nv of dimension at most
k there exists a one-parameter subgroup : C* G such that limt0 (t) ·H = 0.
For example, for G = SL2 and V = Vn, the binary forms of degree n, this
amounts to the question whether every subspace H that consists of forms having
a root of multiplicity greater than n/2. This is indeed the case, as we will see. Furthermore
we settle the question for G = SLn and V = S2(Cn)* (symmetric bilinear forms),
V = 2(Cn)* (skew-symmetric bilinear forms) and G = SL3 and V = S3(C3)*
(ternary cubics).
Part II: Multiplicities in Tensor Monomials:
There exist a lot of formulas to decompose a tensor product of representations
V ⊕ W into a direct sum of irreducible representations with respect to an algebraic
group G. However these formulas usually involve summing over the Weyl-group,
which makes explicit calculations often tedious. When considering multiple tensor
products, i.e. tensor monomials V1⊕n1 ⊕ V2⊕n2 · · · Vr⊕nr, then, even with the
use of descent computers, an explicit decomposition is mostly impossible because
of the complexity that arises. For this reason problems involving tensor monomials
remain challenging. The starting point of this work was the following question
asked by Finkelberg: For which (d1, d2, . . . , dn−1) ∈ Nn−1 does the tensor
monomial Cn⊕d1 ⊕ 2Cn⊕d2 ⊕ 3Cn⊕d3 ⊕· · · ⊕ n−1Cn⊕dn−1 , considered as SLn-representation, contain the trivial representation exactly once? We solve this problem
and some related generalizations. However, representations occuring with multiplicity
one in the decomposition of a tensor monomial V1⊕n1 ⊕ V2⊕n2 · · · Vr⊕nr
are rather rare as we prove that multiplicities of subrepresentations of tensor monomials
grow exponentially with respect to ∑ ni. More precisely, we prove, that if G
is a simple complex group and V1, . . . , Vr and W irreducible non-trivial representations
then there is a constant N and a real number > 1 such that if ∑ ni N
then mult(W, V1⊕n1 ⊕ V2⊕n2 · · · Vr⊕nr) ∑ni unless it is zero.
In its current form, this part is a preprint which evolved from my diploma thesis,
where I solved special cases of the two main results Theorem A and Theorem C.
Part III: The Hilbert Nullcone on Tuples of Matrices and Bilinear Forms:
In this joint work with Jan Draisma we explicitly determine the irreducible components
of the nullcone of the representation of G on M!p, where either G =
SL(W) x SL(V) and M = Hom(V,W) (linear maps), or G = SL(V) and M is one
of the representations S2(V*) (symmetric bilinear forms), 2(V*) (skew bilinear
forms), or V * ⊕ V * (arbitrary bilinear forms). Here V and W are vector spaces over
an algebraically closed field K of characteristic zero. We also answer the question
of when the nullcone in M⊕p is defined by the polarisations of the invariants on M;
typically, this is only the case if either dimV or p is small. A fundamental tool in
our proofs is the Hilbert-Mumford criterion for nilpotency.
This preprint has already been accepted for publication in the Mathematische
Zeitschrift. I mainly contributed to the first problem we solved: counting and describing
the components of the nullcone of the symmetric bilinear forms. Most other cases evolved from this one, however
Rodzina chrześcijańska w „kulturze prowizoryczności”
No full textContemporary culture can be described as provisional and temporary. A provisional culture is closely bound with such ideologies as secularism, individualism, relativism, practical materialism, hedonism and consumerism. These dominant cultural paradigms threaten the Christian understanding of family, hindering its comprehensive development and its ability to fulfil its basic objectives such as mutual welfare of the spouses or giving birth to and raising offspring. A provisional culture poses a threat and a great challenge for the family. Taking into account the fact that each culture is an expression of the human spirit and an environment for building identity and interpersonal relations, a Christian family must not ignore the problems of the contemporary world, but rather undertake a dialogue with a provisional culture aimed at presenting and promoting the Christian model of family, being the first school of faith, preaching and defending a culture of life.Współczesną kulturę można określić mianem prowizorycznej lub tymczasowej. Kultura prowizoryczności ściśle łączy się z prądami ideowymi: sekularyzmem, indywidualizmem, relatywizmem, materializmem praktycznym, hedonizmem i konsumpcjonizmem. Zarysowane dominanty kulturowe godzą w chrześcijańskie rozumienie rodziny, uniemożliwiając jej wszechstronny rozwój i realizację podstawowych celów, jakimi są wzajemne dobro małżonków oraz zrodzenie i wychowanie potomstwa. Kultura prowizoryczności zagraża rodzinie i stanowi dla niej wielkie wyzwanie. W związku z tym, że każda kultura jest wyrazem ludzkiego ducha i środowiskiem budowania tożsamości i relacji międzyludzkich, rodzina chrześcijańska nie może zamknąć się na problemy współczesności, lecz powinna podjąć z kulturą prowizoryczności pewną formę dialogu, którego wyznacznikami są następujące zadania: ukazywać i promować chrześcijański model rodziny, być pierwszą szkołą wiary, głosić i bronić kultury życia.ks. 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Estimation in threshold autoregressive models with a stationary and a unit root regime
No full textThis paper treats estimation in a class of new nonlinear threshold autoregressive models with both a stationary and a unit root regime. Existing literature on nonstationary threshold models have basically focused on models where the nonstationarity can be removed by differencing and/or where the threshold variable is stationary. This is not the case for the process we consider, and nonstandard estimation problems are the result. This paper proposes a parameter estimation method for such nonlinear threshold autoregressive models using the theory of null recurrent Markov chains. Under certain assumptions, we show that the ordinary least squares (OLS) estimators of the parameters involved are asymptotically consistent. Furthermore, it can be shown that the OLS estimator of the coefficient parameter involved in the stationary regime can still be asymptotically normal while the OLS estimator of the coefficient parameter involved in the nonstationary regime has a nonstandard asymptotic distribution. In the limit, the rate of convergence in the stationary regime is asymptotically proportional to n-1/4, whereas it is n-1 in the nonstationary regime. The proposed theory and estimation method are illustrated by both simulated data and a real data example.Autoregressive process; null-recurrent process; semiparametric model; threshold time series; unit root structure.
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