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PubMed1975

Primer on certain elements of medical decision making.

The value of a diagnostic test lies in its ability to detect patients with disease (its sensitivity) and to exclude patients without disease (its specificity). For tests with binary outcomes, these measures are fixed. For tests with a continuous scale of values, various cutoff points can be selected to adjust the sensitivity and specificity of the test to conform with the physician's goals. Principles of statistical decision theory and information theory suggest technics for objectively determining these cutoff points, depending upon whether the physician is concerned with health costs, with financial costs, or with the information content of the test.

Bayes Theorem↗
PubMed2005

Side-chain effects in molecular electronic devices.

We discuss the effect of an abundant structural element of molecules on the transmission probabilities of molecular electronic devices. We show that an attachment of side chains to a molecular conductor may lead to zero transmission probabilities. The gaps in the transmission-probability appear approximately at the eigenvalues of the isolated side chains, provided that the corresponding eigenstates are not localized away from the molecular conductor. Simple Hückel-type calculations serve to illustrate the described effect. Furthermore, we show that complex transmission-probability curves, obtained with Kohn-Sham density-functional theory, also exhibit the described side-chain effect.

Journal Article↗
PubMed2006

Grid-based Thomas-Fermi-Amaldi equation with the molecular cusp condition.

First, the Thomas-Fermi-Amaldi (TFA) equation was formulated with a newly derived condition to remove the singularities at the nuclei, which coincided with the molecular cusp condition. Next, the collocation method was applied to the TFA equation using the grid-based density functional theory. In this paper, the electron densities and the radial probabilities for specific atoms (He, Be, Ne, Mg, Ar, Ca) were found to agree with those from the Thomas-Fermi-Dirac (TFD) method. Total energies for specific atoms (He, Ne, Ar, Kr, Xe, Rn) and molecules (H2,CH4) were also found to be close to those from the Hartree-Fock method using the Pople basis set 6-311G relative to the TFD method. In addition, the computational expense to determine the electron density and its corresponding energy for a large scale structure, such as a carbon nanotube, is shown to be much more efficient compared to the conventional Hartree-Fock method using the 6-31G Pople basis set.

Journal Article↗
PubMed2000

Tests of the ratio rule in categorization.

Many theories of learning and memory (e.g., connectionist, associative, rational, exemplar based) produce psychological magnitude terms as output (i.e., numbers representing the momentary level of some subjective property). Many theories assume that these numbers may be translated into choice probabilities via the ratio rule, also known as the choice axiom (Luce, 1959) or the constant-ratio rule (Clarke, 1957). We present two categorization experiments employing artificial, visual, prototype-structured stimuli constructed from 12 symbols positioned on a grid. The ratio rule is shown to be incorrect for these experiments, given the assumption that the magnitude terms for each category are univariate functions of the number of category-appropriate symbols contained in the presented stimulus. A connectionist winner-take-all model of categorical decision (Wills & McLaren, 1997) is shown to account for our data given the same assumption. The central feature underlying the success of this model is the assumption that categorical decisions are based on a Thurstonian choice process (Thurstone, 1927, Case V) whose noise distribution is not double exponential in form.

Adult↗
PubMed2006

Mechanical and ecophysiological significance of the form of a young Acer rufinerve tree: vertical gradient in branch mechanical properties.

Most tree biomechanics models assume uniformity of mechanical properties within a tree, and only a few studies have focused on differences in mechanical status among branches. We examined mechanical properties of 49 branches of two 10-year-old trees of Acer rufinerve Sieb. et Zucc. For each branch, bending moment due to its own fresh mass, elastic modulus, section modulus and flexural stiffness were obtained. Elastic modulus of the branch was correlated with the density and thickness of the fiber cell wall and decreased with crown depth, indicating that branches at lower positions were more elastic than branches at upper positions. Compared to lower branches, upper branches were less inclined, possessed thicker growth rings, more long shoots and were subject to smaller stresses. The leaf arrangement in the upper branches might be effective in transmitting more light to the lower branches. In contrast, the lower branches were more inclined toward the horizontal and subject to greater stresses than the upper branches. Lower branch inclinations were attributed to smaller dry matter investment in diameter growth. Upper and lower branch inclinations were slightly greater and smaller, respectively, than those predicted by beam theory. The alleviation in inclination of the lower branches is probably caused by negative gravitropic responses such as tension wood formation or upward shoot elongation, or both. The horizontal display of leaves in the lower branches would be effective in light interception. The reduction in cost of the lower branches can be adaptive because they have a shorter life expectancy than the upper branches. The results showed that an adaptive tree form is realized by a vertical gradient in branch mechanical properties.

Acer↗