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As I understand it lme (in R v1.9.x) estimates random effect variances on a log scale, constraining them to be positive. Whilst this seems sensible, it does lead to apparently biased estimates if the variance is actually zero - which makes our simulation results look strange. Whilst we need to think a bit deeper about it - I still haven't got my head around what a negative variance could

...n the 'method' argument of jointModel(). * several types of residuals are supported for the longitudinal and time-to-event outcomes. Moreover, for the longitudinal outcome there is also the option to compute multiple-imputation-based residuals, as described in Rizopoulos, Verbeke and Molenberghs (Biometrics 2009, to appear). * the Weibull submodel for the time-to-event outcome is now available under both the relative risk and accelerated failure time formulations. * this new version of the package features new and more robust algorithms for numerical integration and optimizatio...

...n the 'method' argument of jointModel(). * several types of residuals are supported for the longitudinal and time-to-event outcomes. Moreover, for the longitudinal outcome there is also the option to compute multiple-imputation-based residuals, as described in Rizopoulos, Verbeke and Molenberghs (Biometrics 2009, to appear). * the Weibull submodel for the time-to-event outcome is now available under both the relative risk and accelerated failure time formulations. * this new version of the package features new and more robust algorithms for numerical integration and optimizatio...

...cpress.com/shopping_cart/products/product_detail.asp?sku=C9500> Longitudinal Data Analysis Garrett Fitzmaurice, Brigham and Woman's Hospital, Roxbury, MA, USA Marie Davidian, North Carolina State University, Raleigh, NC, USA Geert Verbeke, Leuven University, Leuven, Belgium Geert Molenberghs, Hasselt University, Diepenbeek, Belgium Publication Date: 6/11/2008 Catalog #: C6587 ISBN: 9781584886587 Number of Pages: 632 Price: $ 89.95/ £ 46.99 Discount Price: $71.96/ £37.59 For more details and to order: http://www.crcpress.com/shopping_cart/p...