On the difference in inference and prediction between the joint and independent f-error models for seemingly unrelated regressions

We consider likelihood and Bayesian inferences for seemingly unrelated (linear) regressions for the joint niultivariate terror (e.g. Zellner, 1976) and the independent t-error (e.g. Maronna, 1976) models. For likelihood inference, the scale matrix and the shape parameter for the joint terror model cannot be consistently estimated because of the lack of adequate information to identify the latter. The joint terror model also yields the same MLEs for the regression coefficients and the scale matrix as for the independent normal error model. which are not robust against outliers. Further, linear hypotheses with respect

to the regression coefficients also give rise to the same mill distributions AS for the independent normal error model, though the MLE has a non-normal limiting distribution. In contrast to the striking similarities between the joint t-error and the independent normal error models, the independent f-error model yields AiLEs that are lubust against uuthers. Since the MLE of the shape parameter reflects the tails of the data distributions, this model extends the independent normal error model for modeling data distributions with relatively t hicker tails. These differences are also discussed with respect to the posterior and predictive distributions for Bayesian inference.     

Extra Care Housing in the UK: Can it be a Home for Life?

Extra care housing is believed to constitute a home for life, precluding the need for institutional accommodations, but currently there is little substantiating evidence. Longitudinal data show that 8.2% of extra care housing residents will move to institutional accommodations after 5 years. Matching analyses suggest that extra care housing residents aged 80 years and older are approximately half as likely to enter institutional care compared with older people in the community in receipt of domiciliary care, albeit with some caveats. Extra care housing may embody a home for life for the majority of residents, although a substantial minority is likely to require institutional care.     

Salinization alters ecosystem structure in urban stormwater detention ponds

Stormwater ponds now comprise a significant portion of standing water in urban areas. These ponds act to sequester excess run-off and pollutants, such as road salt deicers. While these man-made ponds are not intended to serve as freshwater ecosystems, it is becoming clear that they do provide habitat for many organisms, but we know little of their ecosystem structure, function and interactions with pollutants. From April through June 2009, we surveyed 8 stormwater ponds in the Red Run Watershed, Baltimore County, Maryland, USA to describe patterns in aquatic food webs associated with a gradient in road salt inputs. Periphyton, phytoplankton and zooplankton samples were collected from each pond every 4–6 weeks. Specific conductance, a measure of chloride salt loading, among the 8 ponds varied widely throughout the survey, ranging from 99 μS cm⁻¹ to 19,320 μS cm⁻¹. Low (average = 404 +/− 82.6 (SE) μS cm⁻¹) and medium (average = 1749 +/− 267 (SE) μS cm⁻¹) conductance ponds had lower algal biomass and higher densities of zooplankton taxa relative to high (average = 7231 +/− 2143 (SE) μS cm⁻¹) conductance ponds, which were largely devoid of all zooplankton through the end of May. The observed decline in zooplankton density along an increasing chloride gradient is consistent with experimental results of road deicer effects, suggesting algal resources are possibly freed from grazing pressure as zooplankton consumers are negatively impacted by road salt exposure. Our results highlight the potential for both direct and indirect effects of road deicers on freshwater communities.     

Alcohol Advertising in College Newspapers: A 7-Year Follow-up

Abstract

The authors report on the frequency and nature of alcohol advertisements in a representative sample of college newspapers from 1984/85 and compare these with results of a similar study conducted 7 years earlier, in 1977/78. On average, nearly 24 column inches of space per issùe were devoted to national alcohol advertisements in 1984/85. This represents a significant decrease from an earlier period. The space devoted to national alcohol advertisements, however, still far exceeded that for comparison products (books, soft drinks). Local alcohol advertisements averaged 20 column inches per issue and were somewhat more frequent than in the previous study. The content of the advertisements differed markedly from the messages presented 7 years earlier. Although national alcohol advertisements in 1977/78 frequently ridiculed education and study, none did so in 1984/85, when appeals were to taste and product quality instead. Fantasy themes also were relatively common. Very few national alcohol advertisements featured athletes or group drinking. Sponsorship of campus activities by the alcohol industry, however, had increased since the earlier study. In contrast to national advertisements, many local advertisements consisted of inducements to drink through special offers and happy hours. Some encouraged irresponsible and heavy drinking. Interestingly, alcohol advertisements were just as frequent in papers from states with higher minimum drinking ages and were more frequent in papers from campuses with higher proportions of women students.     

Nonuniqueness of least absolute values regression

The question of nonuniqueness of the least absolute values (L₁) regression is discussed, and examples are given of situations where the L₁-regression is unique and where it has 2, 3 and 4 limiting positions. A method is proposed for finding a compromise regression line when the L₁ regression is not unique. Suggestions are made for further research.     

Therapeutic Hypothermia: Quantification of the Transition of Core Body Temperature Using the Flexible Mixture Bent‐Cable Model for Longitudinal Data

Hypothermia which is induced by reducing core body temperature is a therapeutic tool used to prevent brain damage resulting from physical trauma. However, all physiological systems begin to slow down due to hypothermia and this can result in increased risk of mortality. Therefore quantification of the transition of core body temperature to early hypothermia is of great clinical interest. Conceptually core body temperature may exhibit an either gradual or abrupt transition. Bent‐cable regression is an appealing statistical tool to model such data due to the model's flexibility and readily interpretable regression coefficients. It handles more flexibly models that traditionally have been handled by low‐order polynomial models (for gradual transition) or piecewise linear changepoint models (for abrupt change). We consider a rat model to quantify the temporal trend of core body temperature primarily to address the question: What is the critical time point associated with a breakdown in the compensatory mechanisms following the start of hypothermia therapy? To this end, we develop a Bayesian modelling framework for bent‐cable regression of longitudinal data to simultaneously account for gradual and abrupt transitions. Our analysis reveals that: (i) about 39% of rats exhibit a gradual transition in core body temperature; (ii) the critical time point is approximately the same regardless of transition type; and (iii) both transition types show a significant increase of core body temperature followed by a significant decrease.     

A competing risks derivation of a mixture model for the analysis of survival data

Boag (1949) and Berkson and Gage (1952) proposed a mixture model for the analysis of survival time data when aproportion of treated patients are cured. This paper presents a derivation of the Boag/Berkson-Gage mixture model as well as a eneralization of the model based on the theory of competing risks. The assumptions underlying the model are stated and discussed and a general likelihood function is obtained. Use of the model is illustrated ith data from the Stanford Heart Transplant Program.     

Another look at plotting positions

Probability paper was used as early as 1896, and was mentioned in the literature more than 30 times before 1950, mainly by hydrologists, most of whom used the plotting position (i-0.5)/n proposed by Hazen (1914). Gumbel (1942a) considered the modal position (i-1)/(n-1) and the mean position i/(n+1) [the latter proposed by Weibull (1939a,b)], and chose the latter. Lebedev (1952) and others proposed the use of (i-0.3)/(n+0.4), which is approximately the median position advocated by Johnson (1951). Blom (1958) sug-gested (i-α)/(n-2α+1), where a is a constant (usually 0 ≤ α ≤ 1), which includes all of the above plotting positions as special cases. Moreover, by proper choice of α, one can approximate F[E(x_i)], the position proposed by Kimball (1946), for any distri-bution of interest. Gumbel (1954) stated five postulates which plotting positions should satisfy. Chernoff & Lieberman (1954) discussed the optimum choice of plotting positions in various situ-ations. It is clear that the optimum plotting position depends on the use that is to be made of the results and may also depend on the underlying distribution. The author endeavors to formulate recommendations as to the best choice in various situations.     

Classification with discrete and continuous variables via general mixed-data models

We study the problem of classifying an individual into one of several populations based on mixed nominal, continuous, and ordinal data. Specifically, we obtain a classification procedure as an extension to the so-called location linear discriminant function, by specifying a general mixed-data model for the joint distribution of the mixed discrete and continuous variables. We outline methods for estimating misclassification error rates. Results of simulations of the performance of proposed classification rules in various settings vis-à-vis a robust mixed-data discrimination method are reported as well. We give an example utilizing data on croup in children.