160 research outputs found

    Maximum likelihood and pseudo score approaches for parametric time-to-event analysis with informative entry times

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    We develop a maximum likelihood estimating approach for time-to-event Weibull regression models with outcome-dependent sampling, where sampling of subjects is dependent on the residual fraction of the time left to developing the event of interest. Additionally, we propose a two-stage approach which proceeds by iteratively estimating, through a pseudo score, the Weibull parameters of interest (i.e., the regression parameters) conditional on the inverse probability of sampling weights; and then re-estimating these weights (given the updated Weibull parameter estimates) through the profiled full likelihood. With these two new methods, both the estimated sampling mechanism parameters and the Weibull parameters are consistently estimated under correct specification of the conditional referral distribution. Standard errors for the regression parameters are obtained directly from inverting the observed information matrix in the full likelihood specification and by either calculating bootstrap or robust standard errors for the hybrid pseudo score/profiled likelihood approach. Loss of efficiency with the latter approach is considered. Robustness of the proposed methods to misspecification of the referral mechanism and the time-to-event distribution is also briefly examined. Further, we show how to extend our methods to the family of parametric time-to-event distributions characterized by the generalized gamma distribution. The motivation for these two approaches came from data on time to cirrhosis from hepatitis C viral infection in patients referred to the Edinburgh liver clinic. We analyze these data here.Comment: Published in at http://dx.doi.org/10.1214/14-AOAS725 the Annals of Applied Statistics (http://www.imstat.org/aoas/) by the Institute of Mathematical Statistics (http://www.imstat.org

    Modeling the initiation of others into injection drug use, using data from 2,500 injectors surveyed in Scotland during 2008-2009

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    The prevalence of injection drug use has been of especial interest for assessment of the impact of blood-borne viruses. However, the incidence of injection drug use has been underresearched. Our 2-fold aim in this study was to estimate 1) how many other persons, per annum, an injection drug user (IDU) has the equivalent of full responsibility (EFR) for initiating into injection drug use and 2) the consequences for IDUs' replacement rate. EFR initiation rates are strongly associated with incarceration history, so that our analysis of IDUs' replacement rate must incorporate when, in their injecting career, IDUs were first incarcerated. To do so, we have first to estimate piecewise constant incarceration rates in conjunction with EFR initiation rates, which are then combined with rates of cessation from injecting to model IDUs' replacement rate over their injecting career, analogous to the reproduction number of an epidemic model. We apply our approach to Scotland's IDUs, using over 2,500 anonymous injector participants who were interviewed in Scotland's Needle Exchange Surveillance Initiative during 2008-2009. Our approach was made possible by the inclusion of key questions about initiations. Finally, we extend our model to include an immediate quit rate, as a reasoned compensation for higher-than-expected replacement rates, and we estimate how high initiates' quit rate should be for IDUs' replacement rate to be 1

    Commentary : missing targets on drugs-related deaths, and a Scottish paradox

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    The 10-year drug strategy for England and Wales was published in February 2008. It dropped drugs-related deaths (DRDs) as a key performance indicator. Scotland retained a necessary strong focus on DRDs. Scotland's DRDs numbered 1006 in 2000–02 and 1009 in 2003–05. The previous Scottish administration's claim that its number of current injectors had decreased substantially between 2000 and 2003 implied, paradoxically, that their DRD rate would have to have increased. Worse was to come: Scotland's DRDs had increased to 876 in 2006 + 2007. We analyse UK's DRDs by sex and age-group to reveal temporal trends (2000–02 versus 2003–05 versus 2006 + 2007) with different public health and epidemiological implications. We also address the above Scottish paradox and assess, by age-group, how consistent Scotland's 876 DRDs in 2006 + 2007 are with Scottish injectors’ DRD rate in 2003–05 of around 1 per 100 injector-years. Public health success in the UK in reducing DRDs at younger ages should not be overshadowed by the late consequence in terms of older-age DRDs of UK's injector epidemics; in the early 1980s in Scotland, and late 1980s in England and Wales. Targets for reducing DRDs should pay heed to UK's injector epidemics

    Impact of supervision of methadone consumption on deaths related to methadone overdose (1993-2008): analyses using OD4 index in England and Scotland

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    Objective To evaluate the impact of introduction of supervision of methadone dosing on deaths related to overdose of methadone in Scotland and England between 1993 and 2008 while controlling for increased prescribing of methadone
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