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Target Function (target + function)

Distribution by Scientific Domains

Chemistry	50%
Life Sciences	25%

Selected Abstracts

Increasing the Homogeneity of CAT's Item-Exposure Rates by Minimizing or Maximizing Varied Target Functions While Assembling Shadow Tests

JOURNAL OF EDUCATIONAL MEASUREMENT, Issue 3 2005
Yuan H. Li
A computerized adaptive testing (CAT) algorithm that has the potential to increase the homogeneity of CAT's item-exposure rates without significantly sacrificing the precision of ability estimates was proposed and assessed in the shadow-test (van der Linden & Reese, 1998) CAT context. This CAT algorithm was formed by a combination of maximizing or minimizing varied target functions while assembling shadow tests. There were four target functions to be separately used in the first, second, third, and fourth quarter test of CAT. The elements to be used in the four functions were associated with (a) a random number assigned to each item, (b) the absolute difference between an examinee's current ability estimate and an item difficulty, (c) the absolute difference between an examinee's current ability estimate and an optimum item difficulty, and (d) item information. The results indicated that this combined CAT fully utilized all the items in the pool, reduced the maximum exposure rates, and achieved more homogeneous exposure rates. Moreover, its precision in recovering ability estimates was similar to that of the maximum item-information method. The combined CAT method resulted in the best overall results compared with the other individual CAT item-selection methods. The findings from the combined CAT are encouraging. Future uses are discussed. [source]

Nerve growth factor expression in parasympathetic neurons: regulation by sympathetic innervation

EUROPEAN JOURNAL OF NEUROSCIENCE, Issue 12 2000
Wohaib Hasan
Abstract Interactions between sympathetic and parasympathetic nerves are important in regulating visceral target function. Sympathetic nerves are closely apposed to, and form functional synapses with, parasympathetic axons in many effector organs. The molecular mechanisms responsible for these structural and functional interactions are unknown. We explored the possibility that Nerve Growth Factor (NGF) synthesis by parasympathetic neurons provides a mechanism by which sympathetic,parasympathetic interactions are established. Parasympathetic pterygopalatine ganglia NGF-gene expression was examined by in situ hybridization and protein content assessed by immunohistochemistry. Under control conditions, NGF mRNA was present in ,,60% and NGF protein was in 40% of pterygopalatine parasympathetic neurons. Peripheral parasympathetic axons identified by vesicular acetylcholine transporter-immunoreactivity also displayed NGF immunoreactivity. To determine if sympathetic innervation regulates parasympathetic NGF expression, the ipsilateral superior cervical ganglion was excised. Thirty days postsympathectomy, the numbers of NGF mRNA-positive neurons were decreased to 38% and NGF immunoreactive neurons to 15%. This reduction was due to a loss of sympathetic nerve impulse activity, as similar reductions were achieved when superior cervical ganglia were deprived of preganglionic afferent input for 40 days. These findings provide evidence that normally NGF is synthesized by parasympathetic neurons and transported anterogradely to fibre terminals, where it may be available to sympathetic axons. Parasympathetic NGF expression, in turn, is augmented by impulse activity within (and presumably transmitter release from) sympathetic axons. It is suggested that parasympathetic NGF synthesis and its modulation by sympathetic innervation provides a molecular basis for establishment and maintenance of autonomic axo-axonal synaptic interactions. [source]

Improvement of interactions among facts controls

EUROPEAN TRANSACTIONS ON ELECTRICAL POWER, Issue 5 2000
X. Lei
Interaction among the FACTS (flexible AC transmission systems) devices in a multi-machine system can adversely influence the damping properties of individual FACTS devices. In some critical cases, it may even amplify power swings or increase voltage deviations. This paper presents a global tuning method for the FACTS controllers in a multi-machine system. Based on a non-linear optimization algorithm implemented in a simulation program, the parameter settings of the FACTS controllers involved can be simultaneously optimized at transient conditions in coping with the non-linear nature of the power system. By minimizing a non-explicit target function in which swing characteristics between areas are included, interactions among the FACTS controls at transient conditions in the multi-machine system are minimized. Two multi-machine power systems equipped individually with a TCSC (thyristor-controlled series compensator) and a SVC (static Var compensator) are applied to demonstrate the efficiency of the proposed tuning procedure. The results obtained from simulations validate the improvement in damping of overall power oscillations. The simulations also show that the optimized FACTS controllers are robust in providing adequate damping for a range of conditions on the system. [source]

Generic representation and evaluation of properties as a function of position in reciprocal space

JOURNAL OF APPLIED CRYSTALLOGRAPHY, Issue 6 2002
Kevin Cowtan
A generalized approach is described for evaluating arbitrary functions of position in reciprocal space. This is a generalization which subsumes a whole range of calculations that form a part of almost every crystallographic software application. Examples include scaling of structure factors, the calculation of structure-factor statistics, and some simple likelihood calculations for a single parameter. The generalized approach has a number of advantages: all these calculations may now be performed by a single software routine which need only be debugged and optimized once; the existing approach of dividing reciprocal space into resolution shells with discontinuities at the boundaries is no longer necessary; the implementation provided makes employing the new functionality extremely simple and concise. The calculation is split into three standard components, for which a number of implementations are provided for different tasks. A `basis function' describes some function of position in reciprocal space, the shape of which is determined by a small number of parameters. A `target function' describes the property for which a functional representation is required, for example . An `evaluator' takes a basis and target function and optimizes the parameters of the basis function to fit the target function. Ideally the components should be usable in any combination. [source]

Semiautomatic sequence-specific assignment of proteins based on the tertiary structure,The program st2nmr

JOURNAL OF COMPUTATIONAL CHEMISTRY, Issue 3 2002
Primo, Pristov
Abstract The sequence-specific assignment of resonances is still the most time-consuming procedure that is necessary as the first step in high-resolution NMR studies of proteins. In many cases a reliable three-dimensional (3D) structure of the protein is available, for example, from X-ray spectroscopy or homology modeling. Here we introduce the st2nmr program that uses the 3D structure and Nuclear Overhauser Effect spectroscopy (NOESY) peak list(s) to evaluate and optimize trial sequence-specific assignments of spin systems derived from correlation spectra to residues of the protein. A distance-dependent target function that scores trial assignments based on the presence of expected NOESY crosspeaks is optimized in a Monte Carlo fashion. The performance of the program st2nmr is tested on real NMR data of an ,-helical (cytochrome c) and ,-sheet (lipocalin) protein using homology models and/or X-ray structures; it succeeded in completely reproducing the correct sequence-specific assignments in most cases using 2D and/or 15N/13C Nuclear Overhauser Effect (NOE) data. Additionally to amino acid residues the program can also handle ligands that are bound to the protein, such as heme, and can be used as a complementary tool to fully automated assignment procedures. © 2002 Wiley Periodicals, Inc. J Comput Chem 23: 335,340, 2002 [source]

The basis of asymmetry in IS2 transposition

MOLECULAR MICROBIOLOGY, Issue 4 2001
Leslie A. Lewis
In the first step of IS2 transposition, the formation of an IS2 minicircle, the roles of the two IS ends differ. Terminal cleavage initiates exclusively at the right inverted repeat (IRR) , the donor end , whereas IRL is always the target. At the resulting minicircle junction, the two abutted ends are separated by a spacer of 1 or 2 basepairs. In this study, we have identified the determinants of donor and target function. The inability of IRL to act as a donor results largely from two sequence differences between IRL and IRR , an extra basepair between the conserved transposase binding sequences and the end of the element, and a change of the terminal dinucleotide from CA-3, to TA-3,. These two changes also impose a characteristic size on the minicircle junction spacer. The only sequences required for the efficient target function of IRL appear to be contained within the segment from position 11,42. Although IRR can function as a target, its shorter length and additional contacts with transposase (positions 1,7) result in minicircles with longer, and inappropriate, spacers. We propose a model for the synaptic complex in which the terminus of IRL makes different contacts with the transposase for the initial and final strand transfer steps. The sequence differences between IRR and IRL, and the behavioural characteristics of IRL that result from them, have probably been selected because they optimize expression of transposase from the minicircle junction promoter, Pjunc. [source]

A robust bulk-solvent correction and anisotropic scaling procedure

ACTA CRYSTALLOGRAPHICA SECTION D, Issue 7 2005
Pavel V. Afonine
A reliable method for the determination of bulk-solvent model parameters and an overall anisotropic scale factor is of increasing importance as structure determination becomes more automated. Current protocols require the manual inspection of refinement results in order to detect errors in the calculation of these parameters. Here, a robust method for determining bulk-solvent and anisotropic scaling parameters in macromolecular refinement is described. The implementation of a maximum-likelihood target function for determining the same parameters is also discussed. The formulas and corresponding derivatives of the likelihood function with respect to the solvent parameters and the components of anisotropic scale matrix are presented. These algorithms are implemented in the CCTBX bulk-solvent correction and scaling module. [source]

Refinement of protein crystal structures using energy restraints derived from linear-scaling quantum mechanics

ACTA CRYSTALLOGRAPHICA SECTION D, Issue 3 2005
Ning Yu
A novel method is proposed in which combined restraints derived from linear-scaling semiempirical quantum-mechanical (QM) calculations and X-ray diffraction data are combined to refine crystal structures of proteins. Its performance has been tested on a small protein molecule, bovine pancreatic trypsin inhibitor (BPTI). The refinement involves minimization of the sum of a geometric energy function and an X-ray target function based on either the least-squares residual or the maximum-likelihood formalism. For comparison, similar refinement runs have also been performed using energy restraints derived from the force field available in the Crystallography & NMR System (CNS) program. The QM refinements were carried out with weights that were varied by several orders of magnitude and the optimal weights were identified by observing the trend in the final free R values, QM heats of formation and coordinate root-mean-square deviations (r.m.s.d.s) from the crystal structure. It is found that the QM weights are typically smaller but generally on the same scale as the molecular-mechanics (MM) weights for the respective X-ray target functions. The crystallographic R, free R, real-space R values and correlation coefficients based on the structures refined with the energy restraints derived from our QM calculations and Engh and Huber parameters are comparable, suggesting that the QM restraints are capable of maintaining reasonable stereochemistry to a similar degree as the force-field parameters. A detailed inspection of the structures refined with the QM and MM energy restraints reveals that one of the common differences between them and the crystal structure is that the strained bond angles in the crystal structure are corrected after energetically restrained refinements. Systematic differences in certain bond lengths between the QM-refined structures and the statistical averages of experimental structures have also been observed and discussed. [source]

Increasing the Homogeneity of CAT's Item-Exposure Rates by Minimizing or Maximizing Varied Target Functions While Assembling Shadow Tests

Generalized X-ray and neutron crystallographic analysis: more accurate and complete structures for biological macromolecules

ACTA CRYSTALLOGRAPHICA SECTION D, Issue 6 2009
Paul D. Adams
X-ray and neutron crystallographic techniques provide complementary information on the structure and function of biological macromolecules. X-ray and neutron (XN) crystallographic data have been combined in a joint structure-refinement procedure that has been developed using recent advances in modern computational methodologies, including cross-validated maximum-likelihood target functions with gradient-based optimization and simulated annealing. The XN approach for complete (including hydrogen) macromolecular structure analysis provides more accurate and complete structures, as demonstrated for diisopropyl fluorophosphatase, photoactive yellow protein and human aldose reductase. Furthermore, this method has several practical advantages, including the easier determination of the orientation of water molecules, hydroxyl groups and some amino-acid side chains. [source]