Showing papers by "Northwestern University published in 1982"

A Theory of Wage Dynamics

Abstract: A dynamic, equilibrium model of long term (implicit) labour contracts under incomplete but symmetric information is developed. Workers are assumed to be risk averse and of unknown ability or productivity. Risk neutral firms learn, as do workers, about each worker's productivity by observing the worker's output over time. It is shown that equilibrium contracts provide for wages which never decline with age and increase only when the worker's market value increases above his current wage. In addition to characterizing the equilibrium wage contract, we also derive some of its implications for the behaviour of aggregate wages across various groups of workers. These implications explain some findings in the recent empirical literature on ageearnings profiles. In particular our model can explain why earnings may be positively related to experience even after controlling for productivity, as some empirical studies have indicated.

Phenotypic, genetic, and environmental morphological integration in the cranium.

Abstract: Modern theories of evolutionary change have stressed the unity of the genotype (Mayr, 1963, 1976; Lewontin, 1974; Wright, 1978, 1980). It is the total phenotype which is selected upon and the total genotype which evolves rather than individual phenotypic characters or genes. Individual phenotypic characters and genes only evolve within the larger context of the organism in which they occur. Therefore, organisms are the integrated functional units which evolve. This outlook on evolutionary change lends itself to a holistic, systems view of an organism (Waddington, 1957; Riedl, 1978; Gould and Lewontin, 1979). Mayr (1976) refers to the unity of the genotype because most phenotypic characters are the result of the collaboration of many structural and regulatory genes. Most traits of evolutionary importance are polygenic (Lewontin, 1974). Also, most genes are pleiotropic, affecting many different aspects of the phenotype (Wright, 1980). Mayr states, "It is no longer possible to regard the phenotype as a mosaic in which each part can be replaced without any effect on neighboring components . . . (also) the genotype is an organic whole with an internal harmony that, when exposed to a new selection pressure, will change . . . in a harmonious manner" (1976, p. 49). Both genotype and phenotype are systems composed of interacting components.

Contrast sensitivity throughout adulthood

Abstract: Previous studies of spatial contrast sensitivity in adulthood have produced conflicting results. To clarify the situation, we measured contrast sensitivity functions on a large sample of adults (n = 91), ranging in age from 19 to 87. All observers were free from significant ocular pathology and were individually refracted for the test distance. Sensitivity for stationary gratings of low spatial frequency remained the same throughout adulthood. At higher spatial frequencies, sensitivity decreased with age beginning around 40 to 50 years. When a low spatial frequency grating was drifted, young adults' sensitivity improved by a factor of 4-5 over sensitivity to a static grating; this motion enhancement was markedly diminished in adults over 60 years, implying an impairment of temporal processing in the elderly. Reduced retinal illuminance characteristic of the aged eye could account for a large part of older adults' deficit in spatial vision, but appeared to play little role in their deficit in temporal vision.

Flames as gasdynamic discontinuities

Abstract: Early treatments of flames as gasdynamic discontinuities in a fluid flow are based on several hypotheses and/or on phenomenological assumptions. The simplest and earliest of such analyses, by Landau and by Darrieus prescribed the flame speed to be constant. Thus, in their analysis they ignored the structure of the flame, i.e. the details of chemical reactions, and transport processes. Employing this model to study the stability of a plane flame, they concluded that plane flames are unconditionally unstable. Yet plane flames are observed in the laboratory. To overcome this difficulty, others have attempted to improve on this model, generally through phenomenological assumptions to replace the assumption of constant velocity. In the present work we take flame structure into account and derive an equation for the propagation of the discontinuity surface for arbitrary flame shapes in general fluid flows. The structure of the flame is considered to consist of a boundary layer in which the chemical reactions occur, located inside another boundary layer in which transport processes dominate. We employ the method of matched asymptotic expansions to obtain an equation for the evolution of the shape and location of the flame front. Matching the boundary-layer solutions to the outer gasdynamic flow, we derive the appropriate jump conditions across the front. We also derive an equation for the vorticity produced in the flame, and briefly discuss the stability of a plane flame, obtaining corrections to the formula of Landau and Darrieus.

Numbers of receptor sites from Scatchard graphs: facts and fantasies

Abstract: Data for ligand and receptor binding presented in the format of a Scatchard graph are compared with the same data shown as bound ligand plotted against the logarithm of free ligand. From this comparison it is apparent that extrapolations in the Scatchard graph to yield total number of receptor sites are generally not correct.

The Value of Waiting to Invest

Abstract: This paper studies the optimal timing of investment in an irreversible project where the benefits from the project and the investment cost follow continuous-time stochastic processes. The optimal time to invest and an explicit formula for the value of the option to invest are derived. The rule "invest if benefits exceed costs" does not properly account for the option value of waiting.Simulations show that this option value can be significant, and that for surprisingly reasonable parameter values it may be optimal to wait until benefits are twice the investment cost. Finally, we perform comparative static analysis on the valuation formula and on the rule for when to invest.

Medial Axis Transformation of a Planar Shape

Abstract: The medial axis transformation is a means first proposed by Blum to describe a shape. In this paper we present a 0(n log n) algorithm for computing the medial axis of a planar shape represented by an n-edge simple polygon. The algorithm is an improvement over most previously known results interms of both efficiency and exactness and has been implemented in Fortran. Some computer-plotted output of the program are also shown in the paper.

Asymmetric Information, Incentives and Intrafirm Resource Allocation

Abstract: This paper considers the question: How should a firm allocate a resource among divisions when the productivity of the resource in each division is known only to the division manager? Obviously if the divisions as represented by their managers are indifferent among various allocations of the resource, the headquarters can simply request the division managers to reveal their private information on productivity knowing that the managers have no incentive to lie. The resource allocation problem can then be solved under complete or at least symmetric information. This aspect is a flaw in much of the recent literature on this topic, i.e., there is nothing in the models considered which makes divisions prefer one allocation over another. Thus, although in some cases elaborate allocation schemes are proposed and analyzed, they are really unnecessary. In the model we develop, a division can produce the same output with less managerial effort if it is allocated more resources, and effort is costly to the manager. We further assume that this effort is unobservable by the headquarters, so that it cannot infer divisional productivity from data on divisional output and managerial effort. Given these assumptions, we seek an optimal resource allocation process. Our results show that certain types of transfer pricing schemes are optimal. In particular, if there are no potentially binding capacity constraints on production of the resource, then an optimal process is for each division to choose a transfer price from a schedule announced by the headquarters. Division managers receive a fixed compensation minus the cost of the resource allocated to them at the chosen transfer price. Resources are allocated on the basis of the chosen transfer prices. If there is a potentially binding constraint on resource production, a somewhat more complicated, but similar, scheme is required.

Nonlinear theory of film rupture

Abstract: The present work aims at examining nonlinear effects on film rupture by investigating the stability of thin films to finite amplitude disturbances. The dynamics of the liquid film is formulated using the Navier-Stokes equations augmented by a body force describing the London/van der Waals attractions. The liquid film is assumed to be charge neutralized, nondraining, and laterally unbounded. A nonlinear evolution equation is derived for h ( x , t ), the film thickness. This strongly nonlinear partial differential equation is solved by numerical methods as part of an initial-value problem for periodic boundary conditions in x , the lateral space dimension. Given this model, one obtains true rupture in the sense that the film thickness becomes zero in a finite time. The results reveal rupture characteristics and effects of nonlinearities on the rupture properties.

Properties and an Anisotropic Model of Cancellous Bone From the Proximal Tibial Epiphysis

Abstract: An investigation has been made of the source and magnitude of anisotropic material properties of cancellous bone in the proximal epiphysis of the human tibia. Results are reported for stiffness measurements made in three orthogonal directions on 21 cubes of cancellous bone before testing to failure along one of the three principal axes. The structure is approximately transversely isotropic. Strength and stiffness are linear with area fraction for loading along the isotropic axis. Strength is proportional to stiffness for all directions. A finite element model is proposed, based on experimental observations, which enables one to predict the elastic constants of cylindrically structured cancellous bone in the tibia from morphological measurements in the transverse plane.

On k-Nearest Neighbor Voronoi Diagrams in the Plane

Abstract: The notion of Voronoi diagram for a set of N points in the Euclidean plane is generalized to the Voronoi diagram of order k and an iterative algorithm to construct the generalized diagram in 0(k2N log N) time using 0(k2(N − k)) space is presented. It is shown that the k-nearest neighbor problem and other seemingly unrelated problems can be solved efficiently with the diagram.

Time‐dependent self‐consistent field approximation for intramolecular energy transfer. I. Formulation and application to dissociation of van der Waals molecules

Abstract: The time‐dependent self‐consistent field method (TDSCF) is formulated and applied to the study of intramolecular dynamics and unimolecular decomposition processes. The method is illustrated by calculations on vibrational predissociation in van der Waals molecules, such as I2(v)Ne→I2(v′)+Ne. The TDSCF has the advantage of achieving formal separability of the modes by associating a time‐dependent Hamiltonian with each mode, while permitting (possibly extensive) energy transfer among the modes via the time‐dependent mean potential which acts on each mode. We present quantal, semiclassical, and classical versions of the method; a proper classical limit of the quantum TDSCF replaces the averages over wave functions by averages over self‐consistently obtained bundles of trajectories. In all three versions, considerable computational economy is retained in comparison with full dynamics calculation. A detailed study is made of those properties which can be correctly obtained in such a time‐dependent mean field theory. Attention is drawn to problems such as the occurrence of spurious states in the asymptotic region, and a simple method for avoiding them is suggested. We find for the I2(v)Ne vibrational predissociation that the TDSCF compares well with corresponding full dynamics calculations for average single mode properties such as dissociation lifetimes and the translational energy release. Moreover, comparison with classical trajectory calculations shows that the TDSCF method reproduces the essential dynamical mechanism of the dissociation (in‐phase impulsive I⋅⋅⋅Ne collision following several ineffective vibrations). The self‐consistent bundle trajectories and the time‐dependent mean fields are analyzed, and provide insight into the process dynamics. It is concluded that the TDSCF approach in both quantal and quasiclassical versions is a potentially powerful tool in the study of intramolecular energy transfer and unimolecular dissociation.

Volatile production and transport in regional metamorphism

Abstract: Calculations show that H2O and CO2 produced during devolatilization of an average pelite will occupy ∼12 vol. % of the rock at 500°C and 5 kb. Because the tensional strength of well foliated rock at metamorphic conditions is vanishingly small, such a volume of fluid having any vertical extent will fracture the rock and escape upward owing to its lower density.

Efficient and accurate expansion methods for molecules in local density models

Abstract: The effects of truncation of the effective Hamiltonian of local density theory through shape approximations to the molecular charge density are examined. A self‐consistent multicenter‐multipolar (SCM) representation of the density is introduced, which allows calculation of the Coulomb and exchange‐correlation potentials to any desired level of precision. The related question of quality of wave function expansion bases required to reproduce spectra and densities with chemical accuracy is explored. The efficiency and accuracy of the SCM approach are verified by applications to O2 and CO; results are given for the metal cluster compound Ru3(CO)12.

The Personal, Technical, and Public Spheres of Argument: A Speculative Inquiry into the Art of Public Deliberation

Abstract: Argument spheres are symbolic constructions that shape the expectations of interlocutors who engage in the activities of theoretical and practical reasoning. These communicative contexts offer a ra...

Relationships among ontogenetic, static, and evolutionary allometry.

Abstract: The relationship between ontogenetic, static, and evolutionary levels of allometry is investigated. Extrapolation from relative size relationships in adults to relative growth in ontogeny depends on the variability of slopes and intercepts of ontogenetic vectors relative to variability in length of the vector. If variability in slopes and intercepts is low relative to variability in length, ontogenetic and static allometries will be similar. The similarity of ontogenetic and static allometries was tested by comparing the first principal component, or size vector, for correlations among 48 cranial traits in a cross-sectional ontogenetic sample of rhesus macaques from Cayo Santiago with a static sample from which all age- and sex-related variation had been removed. The vector correlation between the components is high but significantly less than one while two of three allometric patterns apparent in the ontogenetic component are not discernable in the static component. This indicates that there are important differences in size and shape relationships among adults and within ontogenies. Extrapolation from intra-or interspecific phenotypic allometry to evolutionary allometry is shown to depend on the similarity of genetic and phenotypic allometry patterns. Similarity of patterns was tested by comparing the first principal components of the phenotypic, genetic, and environmental correlation matrices calculated using standard quantitative genetic methods. The patterns of phenotypic, genetic, and environmental allometry are dissimilar; only the environmental allometries show ontogenetic allometric patterns. This indicates that phenotypic allometry may not be an accurate guide to patterns of evolutionary change in size and shape.

A comparison of men's and women's professional basketball injuries

Abstract: Injuries sustained by male and female professional basketball teams were compared. Injuries from two consecutive seasons were coded, and computer- based cross-tabulations comparing sex, body part, and type of injury were performed. The women's injury frequency was 1.6 times that of men. The body part most frequently injured on both teams was the ankle. Women sustained significantly more knee and thigh injuries as well as sprains, strains, and contusions. Men had significantly more muscle spasms. Other injuries occurred in similar patterns in both sexes. Alterations in training programs are suggested with emphasis on women's strengthening and men's flexi bility.

Updating Subjective Probability

Abstract: Jeffrey's rule for revising a probability P to a new probability P* based on new probabilities P* (Ei ) on a partition {Ei } i = 1 n is P*(A) = Σ P(A| Ei ) P* (Ei ). Jeffrey's rule is applicable if it is judged that P* (A | Ei ) = P(A | Ei ) for all A and i. This article discusses some of the mathematical properties of this rule, connecting it with sufficient partitions, and maximum entropy updating of contingency tables. The main results concern simultaneous revision on two partitions.

The Effect of Treatment on Mortality in Mild Hypertension: Results of the Hypertension Detection and Follow-up Program

Abstract: In the Hypertension Detection and Follow-up Program, 7825 (71.5 per cent) of the 10,940 participants had diastolic blood pressures averaging between 90 and 104 mm Hg on entry into the study and were designated Stratum 1. Half were referred to their usual source of care in the community (the referred-care group), and half were treated intensively in special clinics (the stepped-care group). Five-year mortality in the Stratum 1 patients given stepped care was 20.3 per cent lower than in those given referred care (P less than 0.01). Particularly noteworthy was the beneficial effect of stepped-care treatment on persons with diastolic pressures of 90 to 104 mm Hg who had no evidence of end-organ damage and were not receiving antihypertensive medication when they entered the study. This subgroup had 28.6 per cent fewer deaths at five years among those treated with stepped care than among those treated with referred care (P less than 0.01). These findings support a recommendation that in patients with mild hypertension, treatment should be considered early, before damage to end organs occurs.