Ranked set sampling for ordered categorical variables

The authors show how ranked set sampling, both balanced and unbalanced, can be extended to ordered categorical variables with the goal of estimating the probabilities of all categories. They use ordinal logistic regression to aid in the ranking of the ordinal variable of interest. They also propose an optimal allocation scheme and methods for implementing it under either perfect or imperfect rankings. Results from a simulation study using data from the third National Health and Nutrition Examination Survey indicate that the use of ordinal logistic regression in ranking leads to substantial gains in precision for estimation of cell probabilities.     

Proportional hazards regression in the presence of missing study eligibility information

We consider the study of censored survival times in the situation where the available data consist of both eligible and ineligible subjects, and information distinguishing the two groups is sometimes missing. A complete-case analysis in this context would use only subjects known to be eligible, resulting in inefficient and potentially biased estimators. We propose a two-step procedure which resembles the EM algorithm but is computationally much faster. In the first step, one estimates the conditional expectation of the missing eligibility indicators given the observed data using a logistic regression based on the complete cases (i.e., subjects with non-missing eligibility indicator). In the second step, maximum likelihood estimators are obtained from a weighted Cox proportional hazards model, with the weights being either observed eligibility indicators or estimated conditional expectations thereof. Under ignorable missingness, the estimators from the second step are proven to be consistent and asymptotically normal, with explicit variance estimators. We demonstrate through simulation that the proposed methods perform well for moderate sized samples and are robust in the presence of eligibility indicators that are missing not at random. The proposed procedure is more efficient and more robust than the complete case analysis and, unlike the EM algorithm, does not require time-consuming iteration. Although the proposed methods are applicable generally, they would be most useful for large data sets (e.g., administrative data), for which the computational savings outweigh the price one has to pay for making various approximations in avoiding iteration. We apply the proposed methods to national kidney transplant registry data.     

Modeling restricted mean survival time under general censoring mechanisms

Restricted mean survival time (RMST) is often of great clinical interest in practice. Several existing methods involve explicitly projecting out patient-specific survival curves using parameters estimated through Cox regression. However, it would often be preferable to directly model the restricted mean for convenience and to yield more directly interpretable covariate effects. We propose generalized estimating equation methods to model RMST as a function of baseline covariates. The proposed methods avoid potentially problematic distributional assumptions pertaining to restricted survival time. Unlike existing methods, we allow censoring to depend on both baseline and time-dependent factors. Large sample properties of the proposed estimators are derived and simulation studies are conducted to assess their finite sample performance. We apply the proposed methods to model RMST in the absence of liver transplantation among end-stage liver disease patients. This analysis requires accommodation for dependent censoring since pre-transplant mortality is dependently censored by the receipt of a liver transplant.     

Density zoning and class segregation in U.S. metropolitan areas

Objectives:Socioeconomic segregation rose substantially in U.S. cities during the final decades of the 20th century, and we argue that zoning regulations are an important cause of this increase.Methods. We measure neighborhood economic segregation using the Gini coefficient for neighborhood income inequality and the poor-affluent exposure index. These outcomes are regressed on an index of density zoning developed from the work of Pendall for 50 U.S. metropolitan areas, while controlling for other metropolitan characteristics likely to affect urban housing markets and class segregation.Results. For both 2000 and changes from 1990 to 2000, OLS estimates reveal a strong relationship between density zoning and income segregation, and replication using 2SLS suggests that the relationship is causal. We also show that zoning is associated with higher interjurisdictional inequality.Conclusions. Metropolitan areas with suburbs that restrict the density of residential construction are more segregated on the basis of income than those with more permissive density zoning regimes. This arrangement perpetuates and exacerbates racial and class inequality in the United States.     

Tuning in and hanging out: A preliminary study of college students’ use of podcasts for information,entertainment, and socializing

Millions of listeners tune in to Internet-based audio-on-demand programming (commonly known as audio podcasts). College students represent a large and influential group of consumers but little is known about this primary audience's podcast listening choices and related opinions. This research was organized within the uses and gratifications perspective and involved an online survey of college students who identified as podcast listeners. Results showed most collegiate listeners spent no more than an hour a month listening to podcasts, and fewer than half reported listening to all the episodes they downloaded. Most listeners found podcasts fun and entertaining and said they enjoy sharing what they have heard with peers. Respondents reported clearly identified program genre preferences, and reported ‘multitasking’ in a variety of specific ways while listening. This research does not attempt to answer all questions about the college student podcast audience, but it is a starting point for further study into this important user group. Recommendations for future research are offered.     

Does continuous passive motion during keyboarding affect hand blood flow and wrist function? A prospective case report

The purpose of this prospective case report was to evaluate the use of a keyboard platform device that uses continuous passive motion (CPM) on vascular flow to the hand for clerical employees who perform daily keyboarding tasks. Subjects were two female volunteers, one symptomatic of carpal tunnel syndrome (CTS), who were employed in clerical positions and perform daily keyboarding tasks for most of their workday. Data collection consisted of baseline and follow-up measurements at 6 weeks, including: 1) screening for symptoms based on the Carpal Tunnel Function Disability Form, 2) evaluation using standard physical therapy examination and assessment techniques, including modified Semmes- Weinstein monofilament testing, 3) a typing productivity test, and 4) Doppler ultrasound examination to quantify vascular flow at the wrist. Results revealed that both subjects demonstrated an overall increase in both radial and ulnar blood flow velocity with no decrement in typing productivity. The symptomatic subject also demonstrated an overall improvement of 10 wpm in the typing tests, a decrease in her disability score and symptom severity, and improvement in function. Results suggest that use of CPM as a non-intrusive ergonomic intervention may be used to treat, as well as prevent, carpal tunnel-like symptoms in those who keyboard.     

Distribution of neotropical migratory bird species across an urbanizing landscape

Urbanization leads to long-term modification of landscapes by habitat loss, fragmentation, and the creation of new habitats. Species’ distributions respond to these modifications of habitat availability, but the combination of parameters and scale at which habitat alteration most strongly influences species distributions is poorly understood. We evaluated responses of neotropical migratory birds, a group known to be sensitive to habitat modification, across a gradient of urbanization in the southeastern United States. Thirteen Breeding Bird Survey routes, each with 40 to 50 point counts, were used to quantify species richness across the gradient of urbanization extending from downtown areas of Columbus, GA to natural woodlands. Buffers of 100, 200, and 1000 m radii were constructed from remote images around each counting point to quantify land-use with the goal of evaluating land-use parameters and scales that best described spatial variation in migrant bird species richness. Within each buffer we quantified the proportion of each cover type and within the 1000 m buffers we included several configuration parameters. We used an information-theoretic approach to separate models whose predictor variables were land-use parameters. Because measures of landscape configuration were all correlated with urban cover, these variables were entered separately. In 2002, the best model was composed of large-scale urban cover (negative effect) and mid-scale mixed hardwoods (negative and positive effect) and transitional cover (negative and positive effect) as well as the interaction between the latter two terms (positive effect). In 2003, the best model was composed of weighted edge density (negative effect), mid-scale mixed hardwood cover (negative and positive effect) and large scale transitional cover (positive effect) and the interaction between mixed hardwoods and weighted edge density (positive effect). Our results indicate that large scale habitat attributes influence the local species richness of migrant birds more than smaller scales. These results also indicate that urbanization, through increased urban cover or increasing edge contrast, has strong negative effects on species richness. Our findings support the contention that the conservation value of small woodlots in urban settings may be minimal and suggest that conservation of migratory birds will be best achieved by giving higher priority to sites where urban cover is still low and by preserving large areas of “green space” in urbanizing landscapes. The negative influence of urban cover combined with relatively minor effects of non-urban habitats on distributions of neotropical migratory birds indicates that continued urbanization of landscapes is a serious concern for conservation efforts.     

Child care subsidies and the employment of welfare recipients

Changing patterns of maternal employment, coupled with stronger work requirements for welfare recipients, are increasing the demand for child care. For many families, the cost of child care creates a financial burden; for mothers with low incomes and those who are former welfare recipients, these costs may be an insurmountable barrier to employment or economic self-sufficiency. Despite increased public spending in this area, the receipt of any child care subsidy appears to be a relatively rare and uncertain event. In this study, we use data from a sample of low-income single mothers (current and recent welfare recipients in California) to estimate the probability of their receiving child care subsidies and the effect of this probability on labor market activity.