Getachew Dagne - Academia.edu (original) (raw)

Papers by Getachew Dagne

Introduction: Increased length of time spent in the US is associated with poor dietary habits amo... more Introduction: Increased length of time spent in the US is associated with poor dietary habits among immigrants. Maintaining traditional eating habits from their country of origin is beneficial for disease prevention compared to eating habits associated with migration for many immigrant groups. We examined the role of acculturation and socio-cultural factors in predicting dietary pattern among Jamaican immigrants. Methods: A 2-stage cluster sample design was used to randomly select Jamaicans (25-64 years) in two Florida counties. Ninety-one participants were administered a health questionnaire that assessed acculturation, dietary pattern, and risk factors for CVD. Investigators hypothesized that less acculturated, lower SES, and persons living in ethnic enclaves are more likely to adhere to traditional dietary patterns. Generalized estimating equation modeling was used to examine associations. Results: Persons who were less acculturated were more likely to adhere to a traditional die...

Statistical methods in medical research, Jan 19, 2015

Semicontinuous data featured with an excessive proportion of zeros and right-skewed continuous po... more Semicontinuous data featured with an excessive proportion of zeros and right-skewed continuous positive values arise frequently in practice. One example would be the substance abuse/dependence symptoms data for which a substantial proportion of subjects investigated may report zero. Two-part mixed-effects models have been developed to analyze repeated measures of semicontinuous data from longitudinal studies. In this paper, we propose a flexible two-part mixed-effects model with skew distributions for correlated semicontinuous alcohol data under the framework of a Bayesian approach. The proposed model specification consists of two mixed-effects models linked by the correlated random effects: (i) a model on the occurrence of positive values using a generalized logistic mixed-effects model (Part I); and (ii) a model on the intensity of positive values using a linear mixed-effects model where the model errors follow skew distributions including skew-t and skew-normal distributions (Par...

Archives of Gynecology

OBJECTIVE: To assess the impact of the day of birth on twin mortality in a population sample. MET... more OBJECTIVE: To assess the impact of the day of birth on twin mortality in a population sample. METHODS: We analyzed weekend versus weekday twin births from the United States national twin birth data for the periods 1989-2002. We computed adjusted hazard ratios (HR) and 95 % confidence intervals (CI) to assess the association between infant mortality and weekday of birth using the Cox proportional hazards model. RESULTS: The crude rates for all types of mortality were found to be significantly higher for twins born on weekends than on weekdays. After adjustment, only post-neonatal mortality risk was higher on weekends as compared to weekdays [Hazards ratio (HR) = 1.19, CI: 1.04, 1.36]. Twins of white mothers were at greater risk for neonatal death (HR = 1.16, CI: 1.08, 1.24) but were less likely to experience post-neonatal death (HR = 0.68, CI: 0.64, 0.76) as compared to twins of black mothers. We found an interaction between maternal age and weekday of birth. Twins born on weekends t...

Journal of Biopharmaceutical Statistics, 2014

ABSTRACT In a longitudinal HIV/AIDS study with response data, observations may be missing because... more ABSTRACT In a longitudinal HIV/AIDS study with response data, observations may be missing because of patient dropouts due to drug intolerance or other problems, resulting in non-ignorable missing data. In addition to non-ignorable missingness, there are also problems of skewness and left-censoring in the response variable because of a lower limit of detection (LOD). There has been relatively little work published simultaneously dealing with these features of longitudinal data. In particular, one of the features may sometimes be the existence of a larger proportion of left-censored data falling below LOD than expected under a usually assumed log-normal distribution. When this happens, an alternative model which can account for a high proportion of censored data should be considered. We present an extension of the random effects Tobit model that incorporates a mixture of true undetectable observations and those values from a skew-normal distribution for an outcome with left-censoring, skewness and non-ignorable missingness. A unifying modeling approach is used to assess the impact of left-censoring, skewness, non-ignorable missingness and measurement error in covariates on a Bayesian inference. The proposed methods are illustrated using real data from an AIDS clinical study.

Journal of Thyroid Research, 2014

The obstetric consequences of abnormal thyroid function during pregnancy have been established. L... more The obstetric consequences of abnormal thyroid function during pregnancy have been established. Less understood is the influence of maternal thyroid autoantibodies on infant outcomes. The objective of this study was to examine the influence of maternal thyroperoxidase (TPO) status on fetal/infant brain and body growth. Six-hundred thirty-one (631) euthyroid pregnant women were recruited from prenatal clinics in Tampa Bay, Florida, and the surrounding area between November 2007 and December 2010. TPO status was determined during pregnancy and fetal/infant brain and body growth variables were assessed at delivery. Regression analysis revealed maternal that TPO positivity was significantly associated with smaller head circumference, reduced brain weight, and lower brain-to-body ratio among infants born to TPO+ white, non-Hispanic mothers only, distinguishing race/ethnicity as an effect modifier in the relationship. No significant differences were noted in body growth measurements among infants born to TPO positive mothers of any racial/ethnic group. Currently, TPO antibody status is not assessed as part of the standard prenatal care laboratory work-up, but findings from this study suggest that fetal brain growth may be impaired by TPO positivity among certain populations; therefore autoantibody screening among high-risk subgroups may be useful for clinicians to determine whether prenatal thyroid treatment is warranted.

Statistics in Medicine, 2013

Common problems to many longitudinal HIV/AIDS, cancer, vaccine, and environmental exposure studie... more Common problems to many longitudinal HIV/AIDS, cancer, vaccine, and environmental exposure studies are the presence of a lower limit of quantification of an outcome with skewness and time-varying covariates with measurement errors. There has been relatively little work published simultaneously dealing with these features of longitudinal data. In particular, left-censored data falling below a limit of detection may sometimes have a proportion larger than expected under a usually assumed log-normal distribution. In such cases, alternative models, which can account for a high proportion of censored data, should be considered. In this article, we present an extension of the Tobit model that incorporates a mixture of true undetectable observations and those values from a skew-normal distribution for an outcome with possible left censoring and skewness, and covariates with substantial measurement error. To quantify the covariate process, we offer a flexible nonparametric mixed-effects model within the Tobit framework. A Bayesian modeling approach is used to assess the simultaneous impact of left censoring, skewness, and measurement error in covariates on inference. The proposed methods are illustrated using real data from an AIDS clinical study. .

Statistics in Medicine, 2010

Studies of HIV dynamics in AIDS research are very important in understanding the pathogenesis of ... more Studies of HIV dynamics in AIDS research are very important in understanding the pathogenesis of HIV-1 infection and also in assessing the effectiveness of antiviral therapies. Nonlinear mixed-effects (NLME) models have been used for modeling between-subject and within-subject variations in viral load measurements. Mostly, normality of both within-subject random error and random-effects is a routine assumption for NLME models, but it may be unrealistic, obscuring important features of between-subject and within-subject variations, particularly, if the data exhibit skewness. In this paper, we develop a Bayesian approach to NLME models and relax the normality assumption by considering both model random errors and random-effects to have a multivariate skew-normal distribution. The proposed model provides flexibility in capturing a broad range of non-normal behavior and includes normality as a special case. We use a real data set from an AIDS study to illustrate the proposed approach by comparing various candidate models. We find that the model with skew-normality provides better fit to the observed data and the corresponding estimates of parameters are significantly different from those based on the model with normality when skewness is present in the data. These findings suggest that it is very important to assume a model with skew-normal distribution in order to achieve robust and reliable results, in particular, when the data exhibit skewness.

Tropical Medicine and International Health, 2002

Our aim was to determine the effectiveness of chloroquine prophylaxis in reducing the frequency o... more Our aim was to determine the effectiveness of chloroquine prophylaxis in reducing the frequency of malaria-induced anaemia at delivery. We estimated the haemoglobin levels of 207 parturients; 82 (39.6%) had been on chloroquine prophylaxis [treatment group (TG)] while 125 (60.4%) did not take any malaria preventive medication antenatally [control group (CG)]. The proportion of women with malaria parasitaemia was significantly higher in CG than TG [risk ratio (RR=1.57, CI=1.05-2.34)]. The dose-response relationship between the severity of parasitaemia and the risk of being anaemic (P < 0.001) confirms a strong correlation between gestational malaria and maternal anaemia. There was a 35% reduction in risk for anaemia in the TG compared with the CG (RR=0.65, 0.40-1.06). The difference in risk was more pronounced after adjusting for disparity in place of residence, educational status and obstetric history (adjusted RR=0.54, CI=0.21-0.98). Primiparous mothers appeared to have benefited more from the antianaemic effects of malaria chemoprevention than mothers of higher parity (protective effectiveness 43% compared with 33%, respectively). In conclusion, despite reports of widespread Plasmodium falciparum resistance to chloroquine on the African continent, malaria chemosuppression with the drug was found beneficial in reducing the risk of anaemia at delivery among Cameroonian women.

Journal of Maternal-Fetal and Neonatal Medicine, 2011

The dynamics of fetal programming following in utero demise of a co-twin are poorly understood. T... more The dynamics of fetal programming following in utero demise of a co-twin are poorly understood. The authors examined fetal programming using a unique application of the change-point analysis method, and identified two types of fetal programming that occurred when a viable twin sibling died in utero, while the co-twin survived. In one type, the initial twin fetal programming trajectory was maintained while in a subset of surviving co-twins a "switch" from a twin to a singleton fetal program (dual fetal programming exposure) was observed. The results suggest that the timing in utero of conversion from a twin to a singleton programming pattern occurred slightly earlier among opposite-sex than in same-sex surviving co-twins. For the conversion from a twin to a singleton program to happen, the surviving co-twin must have attained a "critical mass" when the twin sibling died. Whereas, for same-sex surviving co-twins the critical mass for conversion was the 80th percentile of gestational-age specific birth weight, opposite-sex surviving co-twins converted at a lower critical mass (70th percentile). These novel findings warrant further study to confirm the new hithertofore unknown phenomenon of dual fetal programming sequence, and to determine the implications in terms of subsequent morbidity or mortality during infancy, childhood and adult life.

Statistical Methods & Applications, 2014

This article explores Bayesian joint models of event times and longitudinal measures with an atte... more This article explores Bayesian joint models of event times and longitudinal measures with an attempt to overcome departures from normality of the longitudinal response, measurement errors, and shortages of confidence in specifying a parametric time-to-event model. We allow the longitudinal response to have a skew distribution in the presence of measurement errors, and assume the time-to-event variable to have a nonparametric prior distribution. Posterior distributions of the parameters are attained simultaneously for inference based on Bayesian approach. An example from a recent AIDS clinical trial illustrates the methodology by jointly modeling the viral dynamics and the time to decrease in CD4/CD8 ratio in the presence of CD4 counts with measurement errors and to compare potential models with various scenarios and different distribution specifications. The analysis outcome indicates that the time-varying CD4 covariate is closely related to the first-phase viral decay rate, but the time to CD4/CD8 decrease is not highly associated with either the two viral decay rates or the CD4 changing rate over time. These findings may provide some quantitative guidance to better understand the relationship of the virological and immunological responses to antiretroviral treatments.

Psychological Methods, 2002

JOURNAL OF THE JAPAN STATISTICAL SOCIETY, 2012

Journal of Family Psychology, 2005

Observation of interaction plays a central role in family research. This article discusses how to... more Observation of interaction plays a central role in family research. This article discusses how to analyze sequential data generated by discrete microcoding methods to test hypotheses about family interaction. Current methods for studying sequential data are presented, and their limits are discussed. Building on recent applications of contingency table analysis to such data, a multilevel log-linear model is presented that can specify and estimate indicators of individual behavioral tendencies and antecedent-consequent relationships among behaviors, both within and across samples of families. An example of this method is presented using data from a study of couples facing job loss. Potential extensions of this framework for future research are discussed.

Journal of Educational and Behavioral Statistics, 2003

Page 1. Bayesian Hierarchical Modeling of Heterogeneity in Multiple Contingency Tables: An Applic... more Page 1. Bayesian Hierarchical Modeling of Heterogeneity in Multiple Contingency Tables: An Application to Behavioral Observation Data Getachew A. Dagne C. Hendricks Brown University of South Florida George W. Howe The George Washington University ...

Journal of Applied Statistics, 2009

This paper presents a continuous-time Bayesian model for analyzing durations of behavior displays... more This paper presents a continuous-time Bayesian model for analyzing durations of behavior displays in social interactions. Duration data of social interactions are often complex because of repeated behaviors (events) at individual or group (e.g., dyad) level, multiple behaviors (multistates), and several choices of exit from a current event (competing risks). A multilevel, multistate model is proposed to adequately characterize the behavioral processes. The model incorporates dyad-specific and transition-specific random effects to account for heterogeneity among dyads and interdependence among competing risks. The proposed method is applied to child-parent observational data derived from the School Transitions Project to assess the relation of emotional expression in child-parent interaction to risk for early and persisting child conduct problems.

The International Journal of Biostatistics, 2000

Censored data are characteristics of many bioassays in HIV/AIDS studies where assays may not be s... more Censored data are characteristics of many bioassays in HIV/AIDS studies where assays may not be sensitive enough to determine gradations in viral load determination among those below a detectable threshold. Not accounting for such left-censoring appropriately can lead to biased parameter estimates in most data analysis. To properly adjust for left-censoring, this paper presents an extension of the Tobit model for fitting nonlinear dynamic mixed-effects models with skew distributions. Such extensions allow one to specify the conditional distributions for viral load response to account for left-censoring, skewness and heaviness in the tails of the distributions of the response variable. A Bayesian modeling approach via Markov Chain Monte Carlo (MCMC) algorithm is used to estimate model parameters. The proposed methods are illustrated using real data from an HIV/AIDS study.

The International Journal of Biostatistics, 2000

In recent years, various mixed-effects models have been suggested for estimating viral decay rate... more In recent years, various mixed-effects models have been suggested for estimating viral decay rates in HIV dynamic models for complex longitudinal data. Among those models are linear mixed-effects (LME), nonlinear mixed-effects (NLME), and semiparametric nonlinear mixedeffects (SNLME) models. However, a critical question is whether these models produce coherent estimates of viral decay rates, and if not, which model is appropriate and should be used in practice. In addition, one often assumes that a model random error is normally distributed, but the normality assumption may be unrealistic, particularly if the data exhibit skewness. Moreover, some covariates such as CD4 cell count may be often measured with substantial errors. This paper addresses these issues simultaneously by jointly modeling the response variable with skewness and a covariate process with measurement errors using a Bayesian approach to investigate how estimated parameters are changed or different under these three models. A real data set from an AIDS clinical trial study was used to illustrate the proposed models and methods. It was found that there was a significant incongruity in the estimated decay rates in viral loads based on the three mixed-effects models, suggesting that the decay rates estimated by using Bayesian LME or NLME joint models should be interpreted differently from those estimated by using Bayesian SNLME joint models. The findings also suggest that the Bayesian SNLME joint model is preferred to other models because an arbitrary data truncation is not necessary; and it is also shown that the models with a skew-normal distribution and/or measurement errors in covariate may achieve reliable results when the data exhibit skewness.

Infectious Agents and Cancer, 2011

Background: Since behavioral factors are significant determinants of population health, addressin... more Background: Since behavioral factors are significant determinants of population health, addressing prostate cancer (CaP)-related health beliefs and cultural beliefs are key weapons to fight this deadly disease. This study investigated the health beliefs and cultural beliefs of black men relative to CaP, and the key socio-demographic correlates of these beliefs. Methods: The study design was a cross-sectional survey of 2,864 Florida black men, age 40 to 70, on their perceived susceptibility, perceived severity, attitude, outcomes beliefs, perceived behavioral control, CaP fatalism, religiosity, temporal orientation, and acculturation relative to CaP screening and prevention. Results: The men reported favorable attitude and positive outcome beliefs, but moderate perceived behavioral control, CaP susceptibility and CaP severity. They also had low level of acculturation, did not hold fatalistic beliefs about CaP, had high religious coping skills and had high future time perspective. Several demographic variables were found to be associated with health beliefs and cultural beliefs. Discussion: Our study provides rich data with regard to the health and cultural beliefs that might serve to inform the development of CaP control initiative for US-born and foreign-born black men.

Emerging Infectious Diseases, 2001

Early Human Development, 2012

Aims: We used propensity scores matching techniques to assess the association between maternal co... more Aims: We used propensity scores matching techniques to assess the association between maternal cocaine abuse in pregnancy and the occurrence of placenta-associated syndromes (PAS). Study design: Mothers who abused cocaine (n = 5026) were matched to controls (n = 5026) from a sample of 1,693,197, unexposed mothers in Florida from 1998 to 2007. Cocaine abuse was identified using the ICD-9 principal and secondary diagnosis codes (304.2 for cocaine dependence and 305.6 for cocaine abuse). The outcome of interest, PAS, was identified as any indication in diagnosis field of ICD-9-CM codes for: placental abruption (641.2), oligohydramnios (658.0), placental infarction (656.7, 656.8, 656.9), gestational hypertension (642.3, 642.9), preeclampsia (642.4, 642.5, and 642.7) or eclampsia (642.6). Results: Nearly 6% of mothers in the study sample experienced a condition associated with PAS prior to matching. Women who abused cocaine were 58% more likely to have PAS when compared to women who did not (OR = 1.48, 95% confidence interval: 1.33, 1.66). Women who abused cocaine were at elevated odds for placental abruption, placenta infarction and preeclampsia with the most pronounced odds noted for placental abruption (OR = 2.79, 95% confidence interval: 2.19, 3.55). Conclusions: These findings indicate that cocaine abuse during pregnancy is associated with more placentarelated disorders than previously reported.

Introduction: Increased length of time spent in the US is associated with poor dietary habits amo... more Introduction: Increased length of time spent in the US is associated with poor dietary habits among immigrants. Maintaining traditional eating habits from their country of origin is beneficial for disease prevention compared to eating habits associated with migration for many immigrant groups. We examined the role of acculturation and socio-cultural factors in predicting dietary pattern among Jamaican immigrants. Methods: A 2-stage cluster sample design was used to randomly select Jamaicans (25-64 years) in two Florida counties. Ninety-one participants were administered a health questionnaire that assessed acculturation, dietary pattern, and risk factors for CVD. Investigators hypothesized that less acculturated, lower SES, and persons living in ethnic enclaves are more likely to adhere to traditional dietary patterns. Generalized estimating equation modeling was used to examine associations. Results: Persons who were less acculturated were more likely to adhere to a traditional die...

Statistical methods in medical research, Jan 19, 2015

Semicontinuous data featured with an excessive proportion of zeros and right-skewed continuous po... more Semicontinuous data featured with an excessive proportion of zeros and right-skewed continuous positive values arise frequently in practice. One example would be the substance abuse/dependence symptoms data for which a substantial proportion of subjects investigated may report zero. Two-part mixed-effects models have been developed to analyze repeated measures of semicontinuous data from longitudinal studies. In this paper, we propose a flexible two-part mixed-effects model with skew distributions for correlated semicontinuous alcohol data under the framework of a Bayesian approach. The proposed model specification consists of two mixed-effects models linked by the correlated random effects: (i) a model on the occurrence of positive values using a generalized logistic mixed-effects model (Part I); and (ii) a model on the intensity of positive values using a linear mixed-effects model where the model errors follow skew distributions including skew-t and skew-normal distributions (Par...

Archives of Gynecology

OBJECTIVE: To assess the impact of the day of birth on twin mortality in a population sample. MET... more OBJECTIVE: To assess the impact of the day of birth on twin mortality in a population sample. METHODS: We analyzed weekend versus weekday twin births from the United States national twin birth data for the periods 1989-2002. We computed adjusted hazard ratios (HR) and 95 % confidence intervals (CI) to assess the association between infant mortality and weekday of birth using the Cox proportional hazards model. RESULTS: The crude rates for all types of mortality were found to be significantly higher for twins born on weekends than on weekdays. After adjustment, only post-neonatal mortality risk was higher on weekends as compared to weekdays [Hazards ratio (HR) = 1.19, CI: 1.04, 1.36]. Twins of white mothers were at greater risk for neonatal death (HR = 1.16, CI: 1.08, 1.24) but were less likely to experience post-neonatal death (HR = 0.68, CI: 0.64, 0.76) as compared to twins of black mothers. We found an interaction between maternal age and weekday of birth. Twins born on weekends t...

Journal of Biopharmaceutical Statistics, 2014

ABSTRACT In a longitudinal HIV/AIDS study with response data, observations may be missing because... more ABSTRACT In a longitudinal HIV/AIDS study with response data, observations may be missing because of patient dropouts due to drug intolerance or other problems, resulting in non-ignorable missing data. In addition to non-ignorable missingness, there are also problems of skewness and left-censoring in the response variable because of a lower limit of detection (LOD). There has been relatively little work published simultaneously dealing with these features of longitudinal data. In particular, one of the features may sometimes be the existence of a larger proportion of left-censored data falling below LOD than expected under a usually assumed log-normal distribution. When this happens, an alternative model which can account for a high proportion of censored data should be considered. We present an extension of the random effects Tobit model that incorporates a mixture of true undetectable observations and those values from a skew-normal distribution for an outcome with left-censoring, skewness and non-ignorable missingness. A unifying modeling approach is used to assess the impact of left-censoring, skewness, non-ignorable missingness and measurement error in covariates on a Bayesian inference. The proposed methods are illustrated using real data from an AIDS clinical study.

Journal of Thyroid Research, 2014

The obstetric consequences of abnormal thyroid function during pregnancy have been established. L... more The obstetric consequences of abnormal thyroid function during pregnancy have been established. Less understood is the influence of maternal thyroid autoantibodies on infant outcomes. The objective of this study was to examine the influence of maternal thyroperoxidase (TPO) status on fetal/infant brain and body growth. Six-hundred thirty-one (631) euthyroid pregnant women were recruited from prenatal clinics in Tampa Bay, Florida, and the surrounding area between November 2007 and December 2010. TPO status was determined during pregnancy and fetal/infant brain and body growth variables were assessed at delivery. Regression analysis revealed maternal that TPO positivity was significantly associated with smaller head circumference, reduced brain weight, and lower brain-to-body ratio among infants born to TPO+ white, non-Hispanic mothers only, distinguishing race/ethnicity as an effect modifier in the relationship. No significant differences were noted in body growth measurements among infants born to TPO positive mothers of any racial/ethnic group. Currently, TPO antibody status is not assessed as part of the standard prenatal care laboratory work-up, but findings from this study suggest that fetal brain growth may be impaired by TPO positivity among certain populations; therefore autoantibody screening among high-risk subgroups may be useful for clinicians to determine whether prenatal thyroid treatment is warranted.

Statistics in Medicine, 2013

Common problems to many longitudinal HIV/AIDS, cancer, vaccine, and environmental exposure studie... more Common problems to many longitudinal HIV/AIDS, cancer, vaccine, and environmental exposure studies are the presence of a lower limit of quantification of an outcome with skewness and time-varying covariates with measurement errors. There has been relatively little work published simultaneously dealing with these features of longitudinal data. In particular, left-censored data falling below a limit of detection may sometimes have a proportion larger than expected under a usually assumed log-normal distribution. In such cases, alternative models, which can account for a high proportion of censored data, should be considered. In this article, we present an extension of the Tobit model that incorporates a mixture of true undetectable observations and those values from a skew-normal distribution for an outcome with possible left censoring and skewness, and covariates with substantial measurement error. To quantify the covariate process, we offer a flexible nonparametric mixed-effects model within the Tobit framework. A Bayesian modeling approach is used to assess the simultaneous impact of left censoring, skewness, and measurement error in covariates on inference. The proposed methods are illustrated using real data from an AIDS clinical study. .

Statistics in Medicine, 2010

Studies of HIV dynamics in AIDS research are very important in understanding the pathogenesis of ... more Studies of HIV dynamics in AIDS research are very important in understanding the pathogenesis of HIV-1 infection and also in assessing the effectiveness of antiviral therapies. Nonlinear mixed-effects (NLME) models have been used for modeling between-subject and within-subject variations in viral load measurements. Mostly, normality of both within-subject random error and random-effects is a routine assumption for NLME models, but it may be unrealistic, obscuring important features of between-subject and within-subject variations, particularly, if the data exhibit skewness. In this paper, we develop a Bayesian approach to NLME models and relax the normality assumption by considering both model random errors and random-effects to have a multivariate skew-normal distribution. The proposed model provides flexibility in capturing a broad range of non-normal behavior and includes normality as a special case. We use a real data set from an AIDS study to illustrate the proposed approach by comparing various candidate models. We find that the model with skew-normality provides better fit to the observed data and the corresponding estimates of parameters are significantly different from those based on the model with normality when skewness is present in the data. These findings suggest that it is very important to assume a model with skew-normal distribution in order to achieve robust and reliable results, in particular, when the data exhibit skewness.

Tropical Medicine and International Health, 2002

Our aim was to determine the effectiveness of chloroquine prophylaxis in reducing the frequency o... more Our aim was to determine the effectiveness of chloroquine prophylaxis in reducing the frequency of malaria-induced anaemia at delivery. We estimated the haemoglobin levels of 207 parturients; 82 (39.6%) had been on chloroquine prophylaxis [treatment group (TG)] while 125 (60.4%) did not take any malaria preventive medication antenatally [control group (CG)]. The proportion of women with malaria parasitaemia was significantly higher in CG than TG [risk ratio (RR=1.57, CI=1.05-2.34)]. The dose-response relationship between the severity of parasitaemia and the risk of being anaemic (P < 0.001) confirms a strong correlation between gestational malaria and maternal anaemia. There was a 35% reduction in risk for anaemia in the TG compared with the CG (RR=0.65, 0.40-1.06). The difference in risk was more pronounced after adjusting for disparity in place of residence, educational status and obstetric history (adjusted RR=0.54, CI=0.21-0.98). Primiparous mothers appeared to have benefited more from the antianaemic effects of malaria chemoprevention than mothers of higher parity (protective effectiveness 43% compared with 33%, respectively). In conclusion, despite reports of widespread Plasmodium falciparum resistance to chloroquine on the African continent, malaria chemosuppression with the drug was found beneficial in reducing the risk of anaemia at delivery among Cameroonian women.

Journal of Maternal-Fetal and Neonatal Medicine, 2011

The dynamics of fetal programming following in utero demise of a co-twin are poorly understood. T... more The dynamics of fetal programming following in utero demise of a co-twin are poorly understood. The authors examined fetal programming using a unique application of the change-point analysis method, and identified two types of fetal programming that occurred when a viable twin sibling died in utero, while the co-twin survived. In one type, the initial twin fetal programming trajectory was maintained while in a subset of surviving co-twins a "switch" from a twin to a singleton fetal program (dual fetal programming exposure) was observed. The results suggest that the timing in utero of conversion from a twin to a singleton programming pattern occurred slightly earlier among opposite-sex than in same-sex surviving co-twins. For the conversion from a twin to a singleton program to happen, the surviving co-twin must have attained a "critical mass" when the twin sibling died. Whereas, for same-sex surviving co-twins the critical mass for conversion was the 80th percentile of gestational-age specific birth weight, opposite-sex surviving co-twins converted at a lower critical mass (70th percentile). These novel findings warrant further study to confirm the new hithertofore unknown phenomenon of dual fetal programming sequence, and to determine the implications in terms of subsequent morbidity or mortality during infancy, childhood and adult life.

Statistical Methods & Applications, 2014

This article explores Bayesian joint models of event times and longitudinal measures with an atte... more This article explores Bayesian joint models of event times and longitudinal measures with an attempt to overcome departures from normality of the longitudinal response, measurement errors, and shortages of confidence in specifying a parametric time-to-event model. We allow the longitudinal response to have a skew distribution in the presence of measurement errors, and assume the time-to-event variable to have a nonparametric prior distribution. Posterior distributions of the parameters are attained simultaneously for inference based on Bayesian approach. An example from a recent AIDS clinical trial illustrates the methodology by jointly modeling the viral dynamics and the time to decrease in CD4/CD8 ratio in the presence of CD4 counts with measurement errors and to compare potential models with various scenarios and different distribution specifications. The analysis outcome indicates that the time-varying CD4 covariate is closely related to the first-phase viral decay rate, but the time to CD4/CD8 decrease is not highly associated with either the two viral decay rates or the CD4 changing rate over time. These findings may provide some quantitative guidance to better understand the relationship of the virological and immunological responses to antiretroviral treatments.

Psychological Methods, 2002

JOURNAL OF THE JAPAN STATISTICAL SOCIETY, 2012

Journal of Family Psychology, 2005

Observation of interaction plays a central role in family research. This article discusses how to... more Observation of interaction plays a central role in family research. This article discusses how to analyze sequential data generated by discrete microcoding methods to test hypotheses about family interaction. Current methods for studying sequential data are presented, and their limits are discussed. Building on recent applications of contingency table analysis to such data, a multilevel log-linear model is presented that can specify and estimate indicators of individual behavioral tendencies and antecedent-consequent relationships among behaviors, both within and across samples of families. An example of this method is presented using data from a study of couples facing job loss. Potential extensions of this framework for future research are discussed.

Journal of Educational and Behavioral Statistics, 2003

Page 1. Bayesian Hierarchical Modeling of Heterogeneity in Multiple Contingency Tables: An Applic... more Page 1. Bayesian Hierarchical Modeling of Heterogeneity in Multiple Contingency Tables: An Application to Behavioral Observation Data Getachew A. Dagne C. Hendricks Brown University of South Florida George W. Howe The George Washington University ...

Journal of Applied Statistics, 2009

This paper presents a continuous-time Bayesian model for analyzing durations of behavior displays... more This paper presents a continuous-time Bayesian model for analyzing durations of behavior displays in social interactions. Duration data of social interactions are often complex because of repeated behaviors (events) at individual or group (e.g., dyad) level, multiple behaviors (multistates), and several choices of exit from a current event (competing risks). A multilevel, multistate model is proposed to adequately characterize the behavioral processes. The model incorporates dyad-specific and transition-specific random effects to account for heterogeneity among dyads and interdependence among competing risks. The proposed method is applied to child-parent observational data derived from the School Transitions Project to assess the relation of emotional expression in child-parent interaction to risk for early and persisting child conduct problems.

The International Journal of Biostatistics, 2000

Censored data are characteristics of many bioassays in HIV/AIDS studies where assays may not be s... more Censored data are characteristics of many bioassays in HIV/AIDS studies where assays may not be sensitive enough to determine gradations in viral load determination among those below a detectable threshold. Not accounting for such left-censoring appropriately can lead to biased parameter estimates in most data analysis. To properly adjust for left-censoring, this paper presents an extension of the Tobit model for fitting nonlinear dynamic mixed-effects models with skew distributions. Such extensions allow one to specify the conditional distributions for viral load response to account for left-censoring, skewness and heaviness in the tails of the distributions of the response variable. A Bayesian modeling approach via Markov Chain Monte Carlo (MCMC) algorithm is used to estimate model parameters. The proposed methods are illustrated using real data from an HIV/AIDS study.

The International Journal of Biostatistics, 2000

In recent years, various mixed-effects models have been suggested for estimating viral decay rate... more In recent years, various mixed-effects models have been suggested for estimating viral decay rates in HIV dynamic models for complex longitudinal data. Among those models are linear mixed-effects (LME), nonlinear mixed-effects (NLME), and semiparametric nonlinear mixedeffects (SNLME) models. However, a critical question is whether these models produce coherent estimates of viral decay rates, and if not, which model is appropriate and should be used in practice. In addition, one often assumes that a model random error is normally distributed, but the normality assumption may be unrealistic, particularly if the data exhibit skewness. Moreover, some covariates such as CD4 cell count may be often measured with substantial errors. This paper addresses these issues simultaneously by jointly modeling the response variable with skewness and a covariate process with measurement errors using a Bayesian approach to investigate how estimated parameters are changed or different under these three models. A real data set from an AIDS clinical trial study was used to illustrate the proposed models and methods. It was found that there was a significant incongruity in the estimated decay rates in viral loads based on the three mixed-effects models, suggesting that the decay rates estimated by using Bayesian LME or NLME joint models should be interpreted differently from those estimated by using Bayesian SNLME joint models. The findings also suggest that the Bayesian SNLME joint model is preferred to other models because an arbitrary data truncation is not necessary; and it is also shown that the models with a skew-normal distribution and/or measurement errors in covariate may achieve reliable results when the data exhibit skewness.

Infectious Agents and Cancer, 2011

Background: Since behavioral factors are significant determinants of population health, addressin... more Background: Since behavioral factors are significant determinants of population health, addressing prostate cancer (CaP)-related health beliefs and cultural beliefs are key weapons to fight this deadly disease. This study investigated the health beliefs and cultural beliefs of black men relative to CaP, and the key socio-demographic correlates of these beliefs. Methods: The study design was a cross-sectional survey of 2,864 Florida black men, age 40 to 70, on their perceived susceptibility, perceived severity, attitude, outcomes beliefs, perceived behavioral control, CaP fatalism, religiosity, temporal orientation, and acculturation relative to CaP screening and prevention. Results: The men reported favorable attitude and positive outcome beliefs, but moderate perceived behavioral control, CaP susceptibility and CaP severity. They also had low level of acculturation, did not hold fatalistic beliefs about CaP, had high religious coping skills and had high future time perspective. Several demographic variables were found to be associated with health beliefs and cultural beliefs. Discussion: Our study provides rich data with regard to the health and cultural beliefs that might serve to inform the development of CaP control initiative for US-born and foreign-born black men.

Emerging Infectious Diseases, 2001

Early Human Development, 2012

Aims: We used propensity scores matching techniques to assess the association between maternal co... more Aims: We used propensity scores matching techniques to assess the association between maternal cocaine abuse in pregnancy and the occurrence of placenta-associated syndromes (PAS). Study design: Mothers who abused cocaine (n = 5026) were matched to controls (n = 5026) from a sample of 1,693,197, unexposed mothers in Florida from 1998 to 2007. Cocaine abuse was identified using the ICD-9 principal and secondary diagnosis codes (304.2 for cocaine dependence and 305.6 for cocaine abuse). The outcome of interest, PAS, was identified as any indication in diagnosis field of ICD-9-CM codes for: placental abruption (641.2), oligohydramnios (658.0), placental infarction (656.7, 656.8, 656.9), gestational hypertension (642.3, 642.9), preeclampsia (642.4, 642.5, and 642.7) or eclampsia (642.6). Results: Nearly 6% of mothers in the study sample experienced a condition associated with PAS prior to matching. Women who abused cocaine were 58% more likely to have PAS when compared to women who did not (OR = 1.48, 95% confidence interval: 1.33, 1.66). Women who abused cocaine were at elevated odds for placental abruption, placenta infarction and preeclampsia with the most pronounced odds noted for placental abruption (OR = 2.79, 95% confidence interval: 2.19, 3.55). Conclusions: These findings indicate that cocaine abuse during pregnancy is associated with more placentarelated disorders than previously reported.