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Correlation is inherent in longitudinal studies due to the repeated measurements on subjects, as well as due to time-dependent covariates in the study. In the National Longitudinal Study of Adolescent to Adult Health (Add Health), data were repeatedly collected on children in grades 7-12 across four waves. Thus, observations obtained on the same adolescent were correlated, while predictors were correlated with current and future outcomes such as obesity status, among other health issues. Previous methods, such as the generalized method of moments (GMM) approach have been proposed to estimate regression coefficients for time-dependent covariates. However, these approaches combined all valid moment conditions to produce an averaged parameter estimate for each covariate and thus assumed that the effect of each covariate on the response was constant across time. This assumption is not necessarily optimal in applications such as Add Health or health-related data. Thus, we depart from this assumption and instead use the Partitioned GMM approach to estimate multiple coefficients for the data based on different time periods. These extra regression coefficients are obtained using a partitioning of the moment conditions pertaining to each respective relationship. This approach offers a deeper understanding and appreciation into the effect of each covariate on the response. We conduct simulation studies, as well as analyses of obesity in Add Health, rehospitalization in Medicare data, and depression scores in a clinical study. The Partitioned GMM methods exhibit benefits over previously proposed models with improved insight into the nonconstant relationships realized when analyzing longitudinal data., (© 2019 John Wiley & Sons, Ltd.)

Background and Objectives: Complex regional pain syndrome (CRPS) is a poorly understood pain disorder with little information on the natural course of the disease. Changes in its diagnostic criteria have simplified the identification of this syndrome, but convincing epidemiological data regarding this disorder are still lacking. Here, we collected epidemiological and other relevant information regarding CRPS via a Web-based survey to develop a better understanding of the epidemiology, symptoms, progression, therapy, and associated psychosocial factors related to CRPS., Methods: A survey of 75 questions was hosted on the Web site of the Reflex Sympathetic Dystrophy Syndrome Association of America for 5 months. One thousand three hundred fifty-nine subjects responded, and 888 of these satisfied the inclusion criteria for CRPS and were accepted for data analysis., Results: Complex regional pain syndrome affected mostly white women in the 25- to 55-year-old age group. It was often precipitated by trauma (surgical or nonsurgical) and commonly involved the lower ( approximately 56%) and upper ( approximately 38%) extremities. Pain was usually accompanied by edema, vasomotor, sudomotor, motor, and trophic changes. The syndrome commonly progressed and spread to involve other body areas. Affected patients failed multiple pharmacological and nonpharmacological interventions. The syndrome frequently interfered with job ( approximately 62% disability rate), sleep ( approximately 96%), mobility ( approximately 86%), and self-care ( approximately 57%). Remissions and relapses were both common., Conclusions: Complex regional pain syndrome is a severe disabling pain disorder that results in physical as well as emotional and financial consequences to patients. The disease complexity requires coordination of multidisciplinary care that can be achieved by educational efforts directed to general practitioners.