Teenage Childbearing and Its Life Cycle Consequences: Exploiting a Natural Experiment

In this paper, we exploit a 'natural experiment' associated with human reproduction to identify the effect of teen childbearing on subsequent educational attainment, family structure, labor market outcomes and financial self-sufficiency. In particular, we exploit the fact that a substantial fraction of women who become pregnant experience a miscarriage (spontaneous abortion) and thus do not have a birth. If miscarriages were purely random and if miscarriages were the only way, other than by live births, that a pregnancy ended, then women, who had a miscarriage as a teen, would constitute an ideal control group with which to contrast teenage mothers. Exploiting this natural experiment, we devise an Instrumental Variables (IV) estimators for the consequences of teen mothers not delaying their childbearing, using data from the National Longitudinal Survey of Youth, 1979 (NLSY79). Our major finding is that many of the negative consequences of not delaying childbearing until adulthood are much smaller than has been estimated in previous studies. While we do find adverse consequences of teenage childbearing immediately following a teen mother's first birth, these negative consequences appear short- lived. By the time a teen mother reachers her late twenties, she appears to have only slightly more children, is only slightly more likely to be single mother, and has no lower levels of educational attainment than if she had delayed her childbearing to adulthood. In fact, by this age teen mothers appear to be better off in some aspects of their lives. Teenage childbearing appears to raise levels of labor supply, accumulated work experience and labor market earnings and appears to reduce the chances of living in poverty and participating in the associated social welfare programs. These estimated effects imply that the cost of teenage childbearing to U.S. taxpayers is negligible. In particular, our estimates imply that the widely held view that teenage childbearing imposes a substantial cost on government is an artifact of the failure to appropriately account for pre- existing socioeconomic differences between teen mothers and other women when estimating the causal effects of early childbearing. While teen mothers are very likely to live in poverty and experience other forms of adversity, our results imply that little of this would be changed just by getting teen mothers to delay their childbearing into adulthood.

Using permeable membranes to produce hydrogen and oxygen from water

Concept may make it profitable to obtain hydrogen fuel from water. Laboratory tests have demonstrated that method enables decomposition of water several orders of magnitude beyond equilibrium state where only small amounts of free hydrogen are present

An evaluation on the effectiveness of Web 2.0 Startpages (Netvibes & Pageflakes) within NHS libraries.

Carol McCormick was Learning Resources Advisor in the library at James Cook University Hospital, South Teesside when she completed her BSc (Hons) Librarianship (Work Based Learning) degree at Northumbria University. She gained a 1st Class Honours and is now Learning Resources Librarian. Carol’s dissertation formed part of a wider action research project into the provision of current awareness services at James Cook University Hospital. This article reports on the evaluation which was conducted after a Web 2.0 Startpage, or portal, had been introduced to improve access to current awareness information for all staff within the Trust. It is the second article in the Dissertations into practice series to examine the use of web-based tools to improve access to information for NHS staff

Engineering MultiQueues: Fast relaxed concurrent priority queues

Priority queues with parallel access are an attractive data structure for applications like prioritized online scheduling, discrete event simulation, or greedy algorithms. However, a classical priority queue constitutes a severe bottleneck in this context, leading to very small throughput. Hence, there has been significant interest in concurrent priority queues with relaxed semantics. We investigate the complementary quality criteria rank error (how close are deleted elements to the global minimum) and delay (for each element x, how many elements with lower priority are deleted before x). In this paper, we introduce MultiQueues as a natural approach to relaxed priority queues based on multiple sequential priority queues. Their naturally high theoretical scalability is further enhanced by using three orthogonal ways of batching operations on the sequential queues. Experiments indicate that MultiQueues present a very good performance-quality tradeoff and considerably outperform competing approaches in at least one of these aspects. We employ a seemingly paradoxical technique of "wait-free locking" that might be of more general interest to convert sequential data structures to relaxed concurrent data structures

Managing rental car businesses in the new economy: Using a multivariate decision model approach

U.S. rental car organizations are having to modify their business models to adapt to the new economy, which includes increased fuel costs, reduced business and leisure travel, and reduced resale of low mileage rental units. Revenue is negatively impacted due to increased maintenance as a result of higher mileage requirements placed on the rental inventory. Changes in the depreciation allowance on the rental car fleet reduced the potential value of vehicles by requiring fleet operations managers to maintain the fleets for longer periods of time. This article presents a multivariate decision-making model, which used in conjunction with in-house performance indicators, will assist operations managers in understanding specific variables likely to impact rental car revenues and optimize their decisions regarding available assets

Simulation, modelling and development of the metris RCA

In partnership with Metris UK we discuss the utilisation of modelling and simulation methods in the development of a revolutionary 7-axis Robot CMM Arm (RCA). An offline virtual model is described, facilitating pre-emptive collision avoidance and assessment of optimal placement of the RCA relative to scan specimens. Workspace accessibility of the RCA is examined under a range of geometrical assumptions and we discuss the effects of arbitrary offsets resulting from manufacturing tolerances. Degeneracy is identified in the number of ways a given pose may be attained and it is demonstrated how a simplified model may be exploited to solve the inverse kinematics problem of finding the “correct” set of joint angles. We demonstrate how the seventh axis may be utilised to avoid obstacles or otherwise awkward poses, giving the unit greater dexterity than traditional CMMs. The results of finite element analysis and static force modelling on the RCA are presented which provide an estimate of the forces exerted on the internal measurement arm in a range of poses

Characteristics, accuracy and reverification of robotised articulated arm CMMs

VDI article 2617 specifies characteristics to describe the accuracy of articulated arm coordinate measuring machines (AACMMs) and outlines procedures for checking them. However the VDI prescription was written with a former generation of machines in mind: manual arms exploiting traditional touch probe technologies. Recent advances in metrology have given rise to noncontact laser scanning tools and robotic automation of articulated arms – technologies which are not adequately characterised using the VDI specification. In this paper we examine the “guidelines” presented in VDI 2617, finding many of them to be ambiguous and open to interpretation, with some tests appearing even to be optional. The engineer is left significant flexibility in the execution of the test procedures and the manufacturer is free to specify many of the test parameters. Such flexibility renders the VDI tests of limited value and the results can be misleading. We illustrate, with examples using the Nikon RCA, how a liberal interpretation of the VDI guidelines can significantly improve accuracy characterisation and suggest ways in which to mitigate this problem. We propose a series of stringent tests and revised definitions, in the same vein as VDI 2617 and similar US standards, to clarify the accuracy characterisation process. The revised methodology includes modified acceptance and reverification tests which aim to accommodate emerging technologies, laser scanning devices in particular, while maintaining the spirit of the existing and established standards. We seek to supply robust re-definitions for the accepted terms “zero point” and “useful arm length”, pre-supposing nothing about the geometry of the measuring device. We also identify a source of error unique to robotised AACMMs employing laser scanners – the forward-reverse pass error. We show how eliminating this error significantly improves the repeatability of a device and propose a novel approach to the testing of probing error based on statistical uncertainty

Isotopic (δ18O and δ2H) Integrity of Water Samples Collected and Stored by Automatic Samplers

Stable water isotopes are increasingly becoming part of routine monitoring programs that use automatic samplers. The objectives of this study were to quantify the uncertainty in isotope signatures due to the length of sample storage (1–24 d) inside autosamplers over a range of air temperatures (5–35°C) and to evaluate the effectiveness of two evaporation reduction measures (mineral oil and high density polyethylene balls). Results of the laboratory study showed that up to 11.8% of the sample volume evaporated when samples were stored in an autosampler at 35°C for 24 d. To prevent significant water isotope fractionation, samples should be retrieved from autosampler