Future Point In Time Research Articles

Neonatal 6-hydroxydopamine lesions lead to opposing changes in the levels of dopamine receptors and their messenger RNAs.

Previous studies have established that selective damage to the early-developing components of the mesostriatal dopamine system produces profound changes in dopamine D1 receptor-mediated behaviors, while decreasing D1 receptor density. In order to better understand the effects of early intrastriatal 6-hydroxydopamine lesions, we studied the ontogenetic expression (postnatal days 7, 14, 35 and 90) of D1 and D2 receptors, and their corresponding messenger RNAs, in rats that had received intrastriatal 6-hydroxydopamine or vehicle lesions on postnatal day 1. Using receptor autoradiography, significant (P < 0.05) decreases in [3H]SCH 23390 binding to D1 receptors in the rostral and caudal dorsomedial and ventromedial caudate of 6-hydroxydopamine-lesioned animals were evident by postnatal day 7, and remained depressed at all future time points. A significant decrease in D1 receptor concentration occurred in the dorsolateral caudate at later time points (postnatal days 35 and 90). [3H]Spiperone binding to D2 receptor sites was unchanged throughout the entire study. In situ hybridization for D1 and D2 messenger RNA expression showed contrasting results. 6-Hydroxydopamine induced significant decreases of D1 messenger RNA levels in the dorsolateral and dorsomedial caudate by postnatal day 7. By postnatal day 14, messenger RNA expression was significantly elevated in the dorsomedial and ventromedial caudate of the 6-hydroxydopamine group, and remained elevated thereafter. D1 messenger RNA levels became elevated in the lateral caudate at later time points (postnatal days 35 and 90). The opposing changes in D1 receptor concentrations and the messenger RNA encoding the protein did not occur as a consequence of increased transport of D1 receptors to striatonigral terminals. D2 messenger RNA levels in the dorsal caudate were significantly decreased on postnatal day 7, and became higher than controls at postnatal day 14, but were unchanged from controls at later time points. Together, the D1 receptor and D1 messenger RNA findings suggest that the normal relationship between levels of D1 receptor transcript and D1 receptor protein is permanently altered following the early loss of dopamine. In contrast, the results indicate that dopamine plays a minor role in the early postnatal development of the D2 receptor protein and transcript. These findings suggest that dopamine may be involved in the coordinated expression of some dopamine receptors and their corresponding messenger RNAs during development.

Plant growth and reproduction along CO2 gradients: non‐linear responses and implications for community change

AbstractThe effects of rising atmospheric CO2 concentrations on natural plant communities will depend upon the cumulative responses of plant growth and reproduction to gradual, incremental changes in climatic conditions. We analysed published studies of plant responses to elevated CO2 to address whether reproductive and total biomass exhibit similar enhancement to elevated vs. ambient CO2 concentrations, and to assess the patterns of plant response along gradients of CO2 concentrations. In six annual plant species, mean enhancement at double ambient vs. ambient CO2 was 1.13 for total biomass and 1.30 for reproductive biomass. The two measures were significantly correlated, but there was considerable scatter in the relationship, indicating that reproductive responses cannot be consistently predicted from enhancement of total biomass. Along experimental CO2 gradients utilizing three concentrations, there was a great diversity of response patterns, including positive, negative, non‐monotonic and non‐significant (flat) responses. The distribution of response patterns differed for plants grown in stands compared to those grown individually. Positive responses were less frequent in competitive environments, and non‐monotonic responses were more frequent. These results emphasize that interpolation of plant response based on enhancement ratios measured at elevated vs. ambient CO2 concentrations is not sufficient to predict community responses to incremental changes in atmospheric conditions. The consequences of differential response patterns were assessed in a simulation of community dynamics for four species of annual plants. The model illustrates that the final community composition at a future point in time depends critically on both the magnitude and the rate of increase of atmospheric CO2.

Optimal pollution control, irreversibilities, and the value of future information

This paper focuses on the issue of optimal pollution control when either pollution itself is irreversible or when some characteristic of the environmental resource is irreversibly destroyed in the course of growing pollution. It is shown that exhausting the assimilative capacity through too heavy pollution is never optimal unless the rate of social time preference is sufficiently high. The paper also investigates the case that decisions about irreversible developments have to be made under uncertainty today when the decision maker faces the prospect of better information about the irreversible damage at some future point in time. A non-negative quasi-option value is shown to exist as in the Arrow-Fisher-Henry model that relates to natural resource deletion by projects of industrial development.

Optimal Tariffs and Income Taxes under Imperfect and Uncertain Foreign Investment

International investment as well as international trade is now a wide phenomenon, and optimal policies to be selected in the presence of both international trade and investment have been a great concern. Jones [7] and Kemp [9] have done pioneering works in this area of study. Previous studies, however, have mostly postulated that international capital movements are perfect. While perfect capital mobility is expected in the long-run, Bhagwati and Brecher [2] and Brecher and Bhagwati [3] emphasized short-run sluggishness of foreign investment; that is, even in the presence of international differentials in rates of return to capital the amount of foreign capital existing in the host country is fixed in the short-run. A realistic case, however, will be between complete fixity and perfect mobility of foreign capital. This is especially true in the medium-run. In such a case the amount of foreign capital existing in a country at a particular future point in time may be reasonably conceived as uncertain to that country. Uncertainty may arise for various reasons. The case includes uncertainty about the level of the rate of return to capital in the world market, and also uncertainty about production technology employed by foreign investors. These cause amounts of foreign capital to be received by the host country uncertain to it even under perfect international capital mobility. Uncertainties in such a case are relevant when the host country government has a long-run perspective, and is concerned with the long-run consequences of its policies. This paper addresses the open question of levels of optimal policies to be chosen when foreign capital movements are uncertain as well as imperfect, and examines whether they modify or make intact the optimal policies established traditionally. Policies considered here are import tariff and income tax by the host country, since these are prevalent and have attracted much attention in the literature of international trade and investment.

Inspection interval for maximum future reliability using the delay time model

The main problem addressed in this article is the determination of an inspection interval T max, given the number of inspections m − 1, which will result in the maximum reliability at some future point in time t = t ∗ . The reliability model developed by Christer is used in which the notion of delay time is involved, representing the start to eventual failure of an item subject to a fault detectable upon inspection. A numerical procedure is used to solve the model for general delay time density f( h) and time to failure from new density g( y). T max is shown to migrate towards the left hand side of the interval [ t ∗/m, t ∗/(m − 1) ] if the number of inspections is allowed to increase. If both densities are exponential then the optimal inspection interval is shown to be T max = t ∗/m . When costs are incorporated into the model then the problem considered becomes that of determining the optimal inspection interval and number of inspections that will result in the maximum reliability at some future time t ∗ at minimum cost.

Measurement Errors in Census Counts and Estimates of Intercensal Net Migration

The authors present a method for generating confidence intervals around estimates of intercensal net migration made using the life table survival method that incorporate estimates of census measurement errors. The life table survival method applies a life table to a census count to project survivors at some past or future time points. Net migration is then estimated as the difference between the projected number of survivors and the enumerated population at that time. The authors present confidence intervals based on mean square error the sum of the variance and squared bias. It is assumed that random variation in the number of net migrants in an age-sex group is due to random variation in mortality rates and to measurement errors in census counts. The authors illustrate the technique using data from a small area in Alaska. (authors)

Life history costs and consequences of rapid reproductive maturation in female rhesus macaques

Life history theory suggests that reproduction at one point in time involves costs in terms of energy, reduced survival, or probability of reproduction at a future point in time. ln long-lived iteroparous organisms, initiating reproduction at a relatively young age may exact a cost in terms of reduced survivorship, but an early age of first reproduction could be beneficial if it lengthens the breeding lifespan. Data collected over 30 years from one population of rhesus macaques, Macaca mulatta, were analyzed to determine the fertility and survivorship costs of initiating reproduction at a relatively young age. Low population density and high social status increased the chances of accelerating age at first parturition, but high dominance rank was not associated with greater lifetime reproductive success. Rapid reproductive maturation neither reduced short-term survivorship nor decreased lifespan. Fertility costs arose if young females reared a male, but not female, offspring. The fitness consequences of rapid reproductive maturation depend upon longevity, with age at death having a significantly greater impact on lifetime reproductive success than age at first parturition.

A Semidiscrete Model for Water and Solute Movement in Tile‐Drained Soils: 1. Governing Equations and Solution

A finite element model has been developed to simulate solute transport in tile‐drained soil‐aquifer systems. Water flow in the unsaturated zone and to drains in the saturated zone was assumed to be at steady state. The model considers the transport of nonreactive solutes, as well as of reactive solutes whose behavior can be described by a distribution coefficient. The exact‐in‐time numerical solution yields explicit expressions for the concentration field at any future point in time without having to compute concentrations at intermediate times. The semidiscrete method involves the determination of an eigensystem of eigenvalues and eigenvectors of the coefficient matrix. The eigensystem may be complex (i.e., it may have imaginary components) due to asymmetry created by the convection term in the governing convection‐dispersion equation. The proposed approach facilitates long‐term predictions of concentrations in drainage effluents and of salt distributions in soil and groundwater. The accuracy of the model was verified by comparing model results with those based on an analytical solution for two‐dimensional solute transport in groundwater.

Utilization of Temporally Ordered Evidence in Evaluative and Predictive Judgments

90 subjects were presented with 3, 4, or 5 previous semesters of students' academic performance and asked to rate how likely a student is an “above average” student (evaluation group) or rate how likely the student would perform “above average” the following semester (prediction group). Use of evidence at each temporal position was estimated with a linear model, Y = X1 b1 + X2 b2 + X3 b3, + X4 b4 + X5 b5, where Y refers to the subject's judgment, X1 refers to the ith academic performance, and bi to the weight, or contribution of the performance in the ith temporal position. Subjects in the prediction groups used the temporal position of the displayed evidence more than subjects in the evaluation groups for both the 3 and 4 previous semesters' displays but not for the 5-semester display. The results were interpreted as supporting an hypothesis that temporal features of the evidence are minimally relevant unless the task requires involvement of the temporal dimension such as making a prediction to a future point in time.

Optimal Maintenance Policies for Single-Server Queueing Systems Subject to Breakdowns

We consider a single-server queueing system with Poisson arrivals and general service times. While the server is up, it is subject to breakdowns according to a Poisson process. When the server breaks down, we need to repair the server immediately by initiating one of two available repair operations. The operating costs of the system include customer holding costs, repair costs and running costs. The objective is to find a corrective maintenance policy that minimizes the long-run average operating costs of the system. The problem is formulated as a semi-Markov decision process. Under some mild conditions on the repair time and service time distributions and the customer holding cost rate function, we prove that there exists an optimal stationary policy which is monotone, i.e., which is characterized by a single threshold parameter: The stochastically faster repair is initiated if and only if the number of customers in the system exceeds this threshold. We also present an efficient algorithm for the determination of an optimal monotone policy and its average cost. We then extend the problem to allow the system to postpone the repair until some future point in time. We provide a partial characterization of an optimal policy and show that monotone policies are, in general, not optimal. The latter problem also extends the authors' previous work.

The Laplace Transform Galerkin technique for efficient time-continuous solution of solute transport in double-porosity media

A new and efficient technique is presented for numerical solution of solute transport problems in multi-dimensional double-porosity media. The method is based on the Laplace Transform Galerkin (LTG) technique (Sudicky, 1989) and employs a convolution integral in the governing advection-dispersion equation to express the diffusive exchange of solute between the mobile and immobile regions. Because the Laplace transformation is applied to the advection-dispersion equation describing transport in the mobile region, the convolution integral, in addition to the temporal derivative, is effectively removed. After the transformed equation is solved using the Galerkin finite element method, nodal concentrations in the time-domain are obtained using a robust and efficient algorithm for inversion of the Laplace-domain nodal concentrations. The method permits the solution to be evaluated at any future point in time without any need for time steps and permits the use of grids having a relatively coarse spatial discretization while avoiding significant numerical dispersion. Further computational efficiency is achieved for large-grid problems by employing an ORTHOMIN-accelerated iterative solver (Vinsome, 1976) to solve the system of Laplace-domain finite element equations. Influence functions describing the mobile-immobile region solute transfer are presented for the case of slab and sphere geometries for the immobile-fluid zone and for the case of first-order exchange theory, along with expressions which unify the three approaches. The utility of the model is demonstrated by applying it to the problem of solute transport in a sandy-type aquifer having a random, spatially-correlated hydraulic conductivity field and comprised of slightly porous grains that admit intragranular diffusion but do not conduct fluid.

Optimal time to repair a broken server

We consider a single-server queueing system with Poisson arrivals and general service times. While the server is up, it is subject to breakdowns according to a Poisson process. When the server breaks down, we may either repair the server immediately or postpone the repair until some future point in time. The operating costs of the system include customer holding costs, repair costs and running costs. The objective is to find a corrective maintenance policy which minimizes the long-run average operating costs of the system. The problem is formulated as a semi-Markov decision process. Under some mild conditions on the repair time and service time distributions and the customer holding cost rate function, we prove that there exists an optimal stationary policy which is characterized by a single threshold parameter: a repair is initiated if and only if the number of customers in the system exceeds this threshold. We also show how the average cost under such policies may be computed and how an optimal policy may efficiently be determined.

Optimal time to repair a broken server

We consider a single-server queueing system with Poisson arrivals and general service times. While the server is up, it is subject to breakdowns according to a Poisson process. When the server breaks down, we may either repair the server immediately or postpone the repair until some future point in time. The operating costs of the system include customer holding costs, repair costs and running costs. The objective is to find a corrective maintenance policy which minimizes the long-run average operating costs of the system. The problem is formulated as a semi-Markov decision process. Under some mild conditions on the repair time and service time distributions and the customer holding cost rate function, we prove that there exists an optimal stationary policy which is characterized by a single threshold parameter: a repair is initiated if and only if the number of customers in the system exceeds this threshold. We also show how the average cost under such policies may be computed and how an optimal policy may efficiently be determined.

Restoration of the Large Electric Power System Using a Computer Generated Sequence of Target Systems

New techniques for restoration of a failed large power system are presented using decision and control approach (Zaborszky, 1980 b). The proposed method takes advantage of fast digital computing, reliable communications, data base structure, modern system theory to set up an expert computer-operator partnership. The computer establishes target systems for future time points for the most effective use of all equipment expected to be available. Effective computer operations are readily established for this task. Once the operator is presented with target systems which he knows to be viable he can very effectively pick action sequences to reach the target. In turn the computer again can check for consistency and arrange the precise time sequence of individual restoration steps. The result is the on line res t oration plan. During its entire preparation and execution the operator is monitoring the plan and the computer is monitoring its execution to guard against improper sequencing or time delays. Generation of the targets is the key feature of this approach. It is a large scale problem which must be done with high computational efficiency and with sufficient accuracy to make the proposed approach successful. Accordingly, the major part of the paper is devoted to the target generation problem.

Relations between job facet comparisons and employee reactions

This study examined relations between a variety of behavioral and attitudinal reactions and employees' feelings of inequity with regard to four job facets: job complexity, supervisory behavior, compensation, and security. Results showed that employees who felt disadvantaged (underrewarded) on each of the facets were less satisfied than those who felt equitably treated or advantaged (overrewarded) relative to their referents. Only the job complexity dimension related significantly to the behavioral reactions. Employees who felt disadvantaged on this dimension exhibited lower performance and higher absenteeism and turnover than employees who felt equitable or advantaged. The nature of the comparative referents used by employees when contrasting their compensation and job complexity facets also related to the behavioral reactions. For example, employees who used self-future referents (themselves at a future point in time) were most likely to permanently withdraw from the organization. Finally, the research examined employee reactions to various combinations of job facet inequities and to combinations of comparative referents.

The supply and utilization of dental services.

In this article the question is addressed whether regional differences in the supply of dental manpower influences the utilization of dental services. The percentage of the population that visits the dentist, is indeed higher in regions with a higher density of dentists. The number of people that visit a dentist is higher among privately insured patients than among the publicly insured. We have constructed a simple model of the behavior of dentists and patients to find out whether this difference is influenced by the supply of dental manpower. On the assumption that dentists prefer the treatment of privately insured patients we predict a greater difference between publicly and privately insured patients in regions with a lower density of dentists than in regions with a higher density. The data dose not unequivocally support this prediction. A second assumption is that differences between social groups in the chances of uptake of regular treatment influence the behavior of patients on future points in time. To find out whether the limited data we have, support this assumption, we have looked at the differences between privately and publicly insured patients in regions that show an important change in density. The results of this analysis are not as predicted.

Racial Equity or Racial Equality

This study examines the relationship between racial equity in labor market processes and racial equality in future labor market rewards. In particular, a regression standardization procedure is used to project the degree of racial inequality in earnings that would exist among men at various future points in time based on three different sets of assumptions about attainment processes in labor market and educational institutions. The most important results suggest that even if racial discrimination were eliminated immediately in labor market and educational institutions, it would take almost 50 years for the black-white earnings ratio to reach .95. This incompatibility between equity and equality needs to be considered more explicitly both by those who advocate a color-blind labor market and those who advocate preferential treatment for blacks.

The Term of a Risk-Free Security

In the late 1930s, Macaulay [7] and Hicks [6] independently introduced the concept of duration as a measure of the length of a stream of cash flows and a measure of the elasticity of the present value of the stream with respect to a change in the rate of discount, respectively. Approximately 15 years later, Reddington [8] and Heynes and Kirton [5] used the concept to develop interest rate immunization rules for the portfolio management of insurance companies. More recently, Fisher and Weil [1] have developed duration based rules to help investors find investments that will insure them of having some fixed amount of money available at a specific future point in time. In [2], Grove uses duration to link the investor's decision to speculate or immunize with his subjective forecast of future interest rate movements. Finally, Haugen and Wichern [3, 4] show the relationship between duration and the characteristics of bonds and stocks. Also, in analyzing the effect of financial leverage on interest rate risk, they demonstrate how the financial manager can manipulate the capital structure of his firm, so as to render the present value of the common stock insensitive to changes in the rate of interest.

A Multilevel Approach to Planning for Capacity Expansion in Water Resource Systems

Supply and demand models are developed as aids in planning future water resource expenditures. The supply model yields least-cost schedules of projects to meet assumed levels of demand. The demand model, conversely, allocates available water supplies within a regional input-output economy to most efficiently utilize the available resource. As more and more water is made available at various future points in time, the extra amount will be used for less important and productive uses. However, as additional water supply is planned, the costs of supplying the resources become excessively high, since less efficient supply projects must be built. For these reasons, the supply and demand models are placed in a hierarchical framework whereby the supply/demand projections are adjusted so that the additional costs of supply match the additional economic value of the supply. This requires a formal extension of classical benefit-cost analysis, which is referred to as dynamic benefit-cost analysis.

High order stiffly stable composite multistep methods for numerical integration of stiff differential equations

Let $$\dot y$$ =f(y,t) withy(t 0)=y 0 possess a solutiony(t) fort≧t 0. Sett n=t 0+nh, n=1, 2,.... Lety 0 denote the approximate solution ofy(t n) defined by the composite multistep method with $$\dot y_n $$ =f(y n ,t n ) andN=1, 2,.... It is conjectured that the method is stiffly stable with orderp=l for alll≧1 and shown to be so forl=1,..., 25. The method is intrinsically efficient in thatl future approximate solution values are established simultaneously in an iterative solution process with only one function evaluation per iteration for each of thel future time points. Step and order control are easily implemented, in that the approximate solution at only one past point appears in each component multistep formula of the method and in that the local truncation error for the first component multistep formula of the method is easily evaluated as $$T^{[l]} = \frac{h}{{t_{Nl} - t_{(N - 1)l - 1} }}\{ y_{Nl}^{PRED} - y_{Nl} \} ,$$ wherey Nl PRED denotes the value att Nl of the Lagrange interpolating polynomial passing through the pointsy (N−1)l+j att (N−1)l+j withj=−1, 0,...,l − 1.