Quantile Regression Methods for Longitudinal Data with Drop-outs: Application to CD4 Cell Counts of Patients Infected with the Human Immunodeficiency Virus

Patients infected with the human immunodeficiency virus (HIV) generally experience a decline in their CD4 cell count (a count of certain white blood cells). We describe the use of quantile regression methods to analyse longitudinal data on CD4 cell counts from 1300 patients who participated in clinical trials that compared two therapeutic treatments: zidovudine and didanosine. It is of scientific interest to determine any treatment differences in the CD4 cell counts over a short treatment period. However, the analysis of the CD4 data is complicated by drop-outs: patients with lower CD4 cell counts at the base-line appear more likely to drop out at later measurement occasions. Motivated by this example, we describe the use of `weighted' estimating equations in quantile regression models for longitudinal data with drop-outs. In particular, the conventional estimating equations for the quantile regression parameters are weighted inversely proportionally to the probability of drop-out. This approach requires the process generating the missing data to be estimable but makes no assumptions about the distribution of the responses other than those imposed by the quantile regression model. This method yields consistent estimates of the quantile regression parameters provided that the model for drop-out has been correctly specified. The methodology proposed is applied to the CD4 cell count data and the results are compared with those obtained from an `unweighted' analysis. These results demonstrate how an analysis that fails to account for drop-outs can mislead.     

The Continued Importance of Quality Parent–Adolescent Relationships During Late Adolescence

The quality of adolescents' relationships with residential parents has been found to predict many different health and behavioral youth outcomes; strong associations have also been found between these outcomes and family processes, and between relationship quality and family processes. Data from Rounds 1–5 of the National Longitudinal Survey of Youth, 1997 were used to examine hypotheses about the influence of the parent–adolescent relationship on subsequent adolescent mental well‐being and delinquency, as mediated by family processes. Using structural equation modeling, we found that the influence of a positive residential parent–adolescent relationship on better mental well‐being and fewer delinquency was entirely mediated by family routines, parental monitoring, and parental supportiveness, net of sociodemographic controls.     

A Multitude of Voices: Pushcart Prize Winning Publications 1976–2006

The Pushcart Prize, established in 1976, has a well-deserved reputation for highlighting the best in small press publication. The authors examined the first thirty volumes, 1976/1977 through 2006, to identify attributes of the items included in each volume and placed the volumes into five time periods of six volumes each to facilitate trend analysis. In order to identify the most productive publications, titles that had fewer than four selections in the thirty volumes and did not appear in at least two time periods were eliminated. The authors examined: press status as independent or affiliated, state and region where published, and type of work (poetry or other). Finally, highly productive titles were reviewed in WorldCat to determine how frequently these were held in the United States.California, Massachusetts, New York, and Ohio have a continuing, substantial presence in the Prize volumes. Most of the publications included were still active and were affiliated with a larger institution. The three small press titles appearing most frequently were Ploughshares, Paris Review, and American Poetry Review. The Pushcart Prize selections most frequently listed in WorldCat were the Hudson Review, the Paris Review, and the American Poetry Review. Each is held by more than eight hundred U.S. libraries.     

Culture, Context, and Homicide of the Elderly

In recent years, there has been a renewed interest in the criminal victimization of elderly persons. Given that homicides of the elderly are thought to occur primarily in the context of robbery/burglary situations, this study seeks to identify situational as well as structural and cultural factors that influence the murder of older persons. In particular, it has been noted that Southern culture promotes a notion that one should 'respect their elders.' Using conservative Protestant affiliation as a proxy for Southern culture, this study also examines the relationship between Southern culture and homicide of the elderly. Findings show that Southern culture as reflected in conservative Protestant affiliation does not insulate the elderly from victimization.     

Social Return on Investment for Good Foot Delivery

This article provides reflections on a social return on investment (SROI) research process with Good Foot Delivery, a work integration social enterprise in Toronto, Canada, engaged in package delivery and employing people with developmental challenges. The article has three aims: (1) to assess Good Foot's key business inputs, activities, and outputs, and the social impact that these have had on its key stakeholders for the 2013 calendar year; (2) to critically reflect on how the research team arrived at the SROI’s inputs, outputs, outcomes, proxies, and other findings; and (3) to discuss some of the strengths and challenges of the SROI process in practice.     

The role,power and influence of company chairs

This paper develops an analytical framework to depict the heterogeneity that characterises the role of board chair and demonstrate the potential variability in how chairs operate boards and exercise power and influence on strategy, control and resource related tasks at board level. Theories of power and influence, as applied to top management teams and boards of directors, are explicated within the context of contemporary governance practices that are establishing the role of the board chair as distinct to that of the chief executive officer. Specifically, the paper maps sources of power and varying contemporary chair practices, including chair nomenclature (i.e. executive vs. non-executive chairs), chair origin (insider vs. outsider) and chair time (full-time vs. part-time). A number of theoretical chair-power models emerge from this analysis and are subject to empirical analysis using data collected from 160 chairs of 500 FTSE-listed companies. Theoretically and empirically, the paper complements structural approaches to studying boards with attention to behaviour on boards. By linking board structure, board process and the exercise of influence, the study reveals both differences amongst chairs in how they run the board, but also that chairs’ differ in the influence they exert on board-related tasks. Full-time executive chairs exert their greatest influence in strategy and resource dependence tasks whereas part-time, non-executive chairs seem to exert more influence over monitoring and control tasks.     

Facilities and HIV prevention in bathhouse and sex club environments

This project evaluated the extent to which businesses with a primary purpose of providing opportunities for sexual encounters between men (e.g., bathhouses and sex clubs) have implemented strategies that target their customers with important HIV and STD prevention messages. Between October 1996 and February 1997, we conducted structured telephone interviews with 63 businesses throughout the United States in order to describe their facilities and their HIV education and prevention efforts. Types of facilities offered were related to what businesses called themselves and the kinds of sex space they provided. All of the businesses reported that they provided condoms and lubricant on site; 95% provided educational materials such as posters and flyers about HIV/AIDS; and 40% provided HIV testing on site, with half of these also providing some type of STD testing. Although some level of HIV prevention and educational efforts by these businesses are described, further investigation into their efficacy is required.     

Probabilistic Reliability Modeling for Oil Exploration & Production (E&P) Facilities in the Tallgrass Prairie Preserve

The aging domestic oil production infrastructure represents a high risk to the environment because of the type of fluids being handled (oil and brine) and the potential for accidental release of these fluids into sensitive ecosystems. Currently, there is not a quantitative risk model directly applicable to onshore oil exploration and production (E&P) facilities. We report on a probabilistic reliability model created for onshore exploration and production (E&P) facilities. Reliability theory, failure modes and effects analysis (FMEA), and event trees were used to develop the model estimates of the failure probability of typical oil production equipment. Monte Carlo simulation was used to translate uncertainty in input parameter values to uncertainty in the model output. The predicted failure rates were calibrated to available failure rate information by adjusting probability density function parameters used as random variates in the Monte Carlo simulations. The mean and standard deviation of normal variate distributions from which the Weibull distribution characteristic life was chosen were used as adjustable parameters in the model calibration. The model was applied to oil production leases in the Tallgrass Prairie Preserve, Oklahoma. We present the estimated failure probability due to the combination of the most significant failure modes associated with each type of equipment (pumps, tanks, and pipes). The results show that the estimated probability of failure for tanks is about the same as that for pipes, but that pumps have much lower failure probability. The model can provide necessary equipment reliability information for proactive risk management at the lease level by providing quantitative information to base allocation of maintenance resources to high-risk equipment that will minimize both lost production and ecosystem damage.     

Analysis of longitudinal multiple-source binary data using generalized estimating equations

Summary.  We present a multivariate logistic regression model for the joint analysis of longitudinal multiple-source binary data. Longitudinal multiple-source binary data arise when repeated binary measurements are obtained from two or more sources, with each source providing a measure of the same underlying variable. Since the number of responses on each subject is relatively large, the empirical variance estimator performs poorly and cannot be relied on in this setting. Two methods for obtaining a parsimonious within-subject association structure are considered. An additional complication arises with estimation, since maximum likelihood estimation may not be feasible without making unrealistically strong assumptions about third- and higher order moments. To circumvent this, we propose the use of a generalized estimating equations approach. Finally, we present an analysis of multiple-informant data obtained longitudinally from a psychiatric interventional trial that motivated the model developed in the paper.