Modeling Two-state Disease Processes with Random Effects

Many chronic medical conditions are manifested by alternating sojourns in symptom-free and symptomatic states. In many cases, in addition to their relapsing and remitting nature, these conditions lead to worsening disease patterns over time and may exhibit seasonal trends. We develop a mixed-effect two-state model for such disease processes in which covariate effects are modeled multiplicatively on transition intensities. The transition intensities, in turn, are functions of three time scales: the semi-Markov scale involving the backward recurrence time for the cyclical component, the Markov scale for the time trend component, and a seasonal time scale. Multiplicative bivariate log-normal random effects are introduced to accommodate heterogeneity in disease activity between subjects and to admit a possible negative correlation between the transition intensities. Maximum likelihood estimation is carried out using Gauss-Hermite integration and a standard Newton-Raphson procedure. Tests of homogeneity are presented based on score statistics. An application of the methodology to data from a multi-center clinical trial of chronic bronchitis is provided for illustrative purposes.     

Mentors for adolescents in foster care

Significant numbers of adolescents in foster placement plan to live independently following discharge. Mentoring is increasingly being used as a support service to assist older foster youths to make the transition to adult living. A survey of 29 child welfare programs indicate that a variety of mentoring models are in use. The main models are transitional Life Skills Mentors, Cultural-Empowerment Mentors, and Corporate-Business Mentors. Mentoring connects foster wards with a cross-section of community citizens who provide a bridge to higher education and employment, and serve as a resource for transitional problem-solving. Information is needed on the impacts of mentor-mentee matches and efforts to sustain mentor-mentee relationships.Based on a paper presented at the Second National Child Welfare Conference, Division of Child Welfare Research, Arlington, Virginia, 1994. The research is supported by Research Grant #90CW1026 entitled Mentors as Resources for Preparing Foster Youths for Independent Living, from the Department of Health and Human Services, Administration for Children and Families, Washington, D.C.the Principal Investigator, Foster Youth Project and Adolescent Mentor Project.     

Robust inference for bivariate point processes

In the analysis of recurrent events where the primary interest lies in studying covariate effects on the expected number of events occurring over a period of time, it is appealing to base models on the cumulative mean function (CMF) of the processes (Lawless & Nadeau 1995). In many chronic diseases, however, more than one type of event is manifested. Here we develop a robust inference procedure for joint regression models for the CMFs arising from a bivariate point process. Consistent parameter estimates with robust variance estimates are obtained via unbiased estimating functions for the CMFs. In most situations, the covariance structure of the bivariate point processes is difficult to specify correctly, but when it is known, an optimal estimating function for the CMFs can be obtained. As a convenient model for more general settings, we suggest the use of the estimating functions arising from bivariate mixed Poisson processes. Simulation studies demonstrate that the estimators based on this working model are practically unbiased with robust variance estimates. Furthermore, hypothesis tests may be based on the generalized Wald or generalized score tests. Data from a trial of patients with bronchial asthma are analyzed to illustrate the estimation and inference procedures.     

Potency Factors for Risk Assessment at Libby,Montana

We reanalyzed the Libby vermiculite miners’ cohort assembled by Sullivan to estimate potency factors for lung cancer, mesothelioma, nonmalignant respiratory disease (NMRD), and all‐cause mortality associated with exposure to Libby fibers. Our principal statistical tool for analyses of lung cancer, NMRD, and total mortality in the cohort was the time‐dependent proportional hazards model. For mesothelioma, we used an extension of the Peto formula. For a cumulative exposure to Libby fiber of 100 f/mL‐yr, our estimates of relative risk (RR) are as follows: lung cancer, RR = 1.12, 95% confidence interval (CI) =[1.06, 1.17]; NMRD, RR = 1.14, 95% CI =[1.09, 1.18]; total mortality, RR = 1.06, 95% CI =[1.04, 1.08]. These estimates were virtually identical when analyses were restricted to the subcohort of workers who were employed for at least one year. For mesothelioma, our estimate of potency is K_M = 0.5 × 10^?8, 95% CI =[0.3 × 10^?8, 0.8 × 10^?8]. Finally, we estimated the mortality ratios standardized against the U.S. population for lung cancer, NMRD, and total mortality and obtained estimates that were in good agreement with those reported by Sullivan. The estimated potency factors form the basis for a quantitative risk assessment at Libby.     

Using simulation to develop testable functionalist explanations: a case study of church survival

Many arguments for and against functionalism have revolved around logical and methodological issues, discouraging attempts to develop testable functionalist theories. This paper takes a different approach, arguing that the problem with sociological functionalism is not its logic of explanation but the absence of appropriate techniques for formalizing functionalist theories. The solution presented --agent-based simulation --is illustrated by a case study: a functionalist account of church survival. The case study suggests that there is nothing wrong with the logic of functionalism properly formalized, illustrates how simulation can enable testing of functionalist explanations and suggests why existing sociological methods have not proved fruitful for this task. In the light of new techniques like simulation, the current neglect of functionalism may no longer be justified.     

Planning for corporate social programs—Problems and guidelines

There is mounting evidence that one of the principal impediments to the increased social responsiveness of major corporations is the organizational difficulty of incorporating novel social programmes into existing operations. Four empirical studies that support this thesis are summarized and discussed. Then, a set of guidelines are suggested that will provide a useful checklist in planning for the implementation of new socially oriented programmes in large firms.     

Use of Acute Toxicity to Estimate Carcinogenic Risk

Data on the effects of human exposure to carcinogens are limited, so that estimation of the risks of carcinogens must be obtained indirectly. Current risk estimates are generally based on lifetime animal bioassays which are expensive and which take more than two years to complete. We here show how data on acute toxicity can be used to make a preliminary estimate of carcinogenic risk and give an idea of the uncertainty in that risk estimate. The estimates obtained are biased upwards, and so are useful for setting interim standards and determining whether further study is worthwhile. A general scheme which incorporates the use of such estimates is outlined, and it is shown by example how adoption of the procedures suggested could have prevented regulatory hiatus in the past.     

Surprised By Love: The Dramatic Structure and Popular Appeal of the Wakefield Second Shepherds' Pageant

The Wakefield Shepherds' Pageant has been one of the most discussed of medieval dramas and deservedly so. Edmund Taft's fresh and insightful essay provides a provocative view o f the play as popular drama, produced by the middle-class and largely for the middle class. The popular appeal of this drama rests in the technique of dramatic surprise, used so effectively by the Wakefield Master to focus on such contemporary social and moral issues as oppression of shepherds, farmers and hired hands, unfeeling masters, unwanted children and marriages of contract only. The shepherds' play, with its powerful spiritual message of love and charity, reflects not only the Church which directed the drama but also the contemporary world of those medieval burghers who produced it.