Flexible semiparametric mixed models

In linear mixed models the influence of covariates is restricted to a strictly parametric form. With the rise of semi- and nonparametric regression also the mixed model has been expanded to allow for additive predictors. The common approach uses the representation of additive models as mixed models. An alternative approach that is proposed in the present paper is likelihood based boosting. Boosting originates in the machine learning community where it has been proposed as a technique to improve classification procedures by combining estimates with reweighted observations. Likelihood based boosting is a general method which may be seen as an extension of L2 boost. In additive mixed models the advantage of boosting techniques in the form of componentwise boosting is that it is suitable for high dimensional settings where many influence variables are present. It allows to fit additive models for many covariates with implicit selection of relevant variables and automatic selection of smoothing parameters. Moreover, boosting techniques may be used to incorporate the subject-specific variation of smooth influence functions by specifying random slopes on smooth e ects. This results in flexible semiparametric mixed models which are appropriate in cases where a simple random intercept is unable to capture the variation of e ects across subjects

Smoothing sparse and unevenly sampled curves using semiparametric mixed models: An application to online auctions

Functional data analysis can be challenging when the functional objects are sampled only very sparsely and unevenly. Most approaches rely on smoothing to recover the underlying functional object from the data which can be difficult if the data is irregularly distributed. In this paper we present a new approach that can overcome this challenge. The approach is based on the ideas of mixed models. Specifically, we propose a semiparametric mixed model with boosting to recover the functional object. While the model can handle sparse and unevenly distributed data, it also results in conceptually more meaningful functional objects. In particular, we motivate our method within the framework of eBay's online auctions. Online auctions produce monotonic increasing price curves that are often correlated across two auctions. The semiparametric mixed model accounts for this correlation in a parsimonious way. It also estimates the underlying increasing trend from the data without imposing model-constraints. Our application shows that the resulting functional objects are conceptually more appealing. Moreover, when used to forecast the outcome of an online auction, our approach also results in more accurate price predictions compared to standard approaches. We illustrate our model on a set of 183 closed auctions for Palm M515 personal digital assistants

Case histories of infectious disease management in developing countries: Phnom Penh and Kabul As doenças infecciosas nos países em desenvolvimento: Phnom Penh e Cabul

Healthcare in developing countries is affected by severe poverty, political instability and diseases that may be of lesser importance in industrialized countries. The aim of this paper was to present two cases and histories of physicians working in hospitals in developing countries and to discuss the opportunities for clinical investigation and collaboration. Cases of patients in Phnom Penh, Cambodia, with histoplasmosis, cryptococcal meningitis, crusted scabies, cerebral lesions and human immunodeficiency virus and of patients in Kabul, Afghanistan, with liver cirrhosis, nephrotic syndrome and facial ulcer are discussed. Greater developmental support is required from industrialized nations, and mutually beneficial cooperation is possible since similar clinical problems exist on both sides (e.g. opportunistic cardiovascular infections). Examples for possible support of hospital medicine include physician interchange visits with defined objectives (e.g. infection control or echocardiography training) and collaboration with clinical investigations and projects developed locally (e.g. epidemiology of cardiovascular diseases or nosocomial bloodborne infections).<br>A assistência à saúde em países em desenvolvimento é afetada pela pobreza extrema, instabilidade política e doenças que podem ter menor importância em países industrializados. O objetivo deste trabalho foi apresentar dois casos e histórias de médicos que trabalham em hospitais de países em desenvolvimento e discutir as oportunidades de investigação clínica e cooperação. São discutidos casos de pacientes em Phnom Penh, no Camboja, com histoplasmose, meningite criptocócica, sarna, lesões cerebrais e vírus da imunodeficiência humana, e de pacientes em Kabul, no Afeganistão, com cirrose hepática, síndrome nefrótica e úlcera facial. Maior apoio ao desenvolvimento por parte dos países desenvolvidos é essencial, e uma cooperação mutuamente benéfica é possível, visto que problemas clínicos similares existem em ambos os lados (p. ex. infecções cardiovasculares oportunistas). Exemplos para possível apoio à medicina hospitalar incluem intercâmbio de médicos para visitas com objetivos definidos (p. ex. controle de infecção ou treinamento em ecocardiografia) e colaboração com investigações clínicas e projetos desenvolvidos localmente (p. ex. epidemiologia de doenças cardiovasculares ou infecções hospitalares causadas por via sanguínea)