Browsing by Author "Phillips, Charles D."

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    Modeling covariance structure in unbalanced longitudinal data
    (2009-05-15) Chen, Min; Huang, Jianhua; Phillips, Charles D.; Sherman, Michael; Wang, Suojin
    Modeling covariance structure is important for efficient estimation in longitudinal data models. Modified Cholesky decomposition (Pourahmadi, 1999) is used as an unconstrained reparameterization of the covariance matrix. The resulting new parameters have transparent statistical interpretations and are easily modeled using covariates. However, this approach is not directly applicable when the longitudinal data are unbalanced, because a Cholesky factorization for observed data that is coherent across all subjects usually does not exist. We overcome this difficulty by treating the problem as a missing data problem and employing a generalized EM algorithm to compute the ML estimators. We study the covariance matrices in both fixed-effects models and mixed-effects models for unbalanced longitudinal data. We illustrate our method by reanalyzing Kenwards (1987) cattle data and conducting simulation studies.
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    Multidrug-Resistant Organism Infections in US Nursing Homes: A National Study of Prevalence, Onset, and Transmission across Care Settings, October 1, 2010-December 31, 2011
    (Infection Control and Hospital Epidemiology, 2014) Kahvecioglu, Daver; Ramiah, Kalpana; McMaughan, Darcy; Garfinkel, Steven; McSorley, Veronica E.; Nguyen, Quy Nhi; Yang, Manshu; Pugliese, Christopher; Mehr, David; Phillips, Charles D.
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    Patterns and predictors of mental health service use and serious mental illness among community-dwelling elderly
    (Texas A&M University, 2006-10-30) Karlin, Bradley Eric; Duffy, Michael; Gleaves, David H.; Morey, Leslie C.; Phillips, Charles D.
    Older adults have historically utilized mental health services at substantially low rates. Unfortunately, though professional, policy, and other recent developments portend an increase in service use, there has been scant empirical attention devoted to the current or recent utilization of mental health treatment by the elderly, and almost nothing is known about the correlates of mental health need and service use among older adults. Accordingly, the present study examined patterns of serious mental illness (SMI), specific mental health syndromes, and service use among older (65+) and younger (18- 64) adults throughout the United States, and the extent to which various factors predict mental health need and the use and magnitude of mental health treatment. In addition, the study examined factors related to unmet need, as well as age group differences in perceived benefit from treatment. The findings reveal that older adults were three times less likely than their younger counterparts to receive any outpatient mental health treatment. Only 2.5% of older individuals utilized any outpatient mental health service in the past year, versus 7.0% of younger adults. The results indicate that the low rate of utilization by older adults may be partly a function of limited subjective mental health need. Prevalence estimates for SMI and all specific mental health syndromes, with the exception of agoraphobia, were markedly lower in the older than the younger cohort. Importantly, though mental health problems appear to be significantly undertreated in older and younger age groups, the study also reveals that those older and younger adults that make it into services typically benefit considerably from treatment. It is hoped that the knowledge yielded by the current study will promote efforts to enhance mental health care access and reduce the long neglected mental health needs of the nation’s elderly population. Several factors related to mental health need and service use were identified in the study that may assist policy, planning, and outreach efforts aimed at increasing service access.