Specific Aim #3: To evaluate whether the effect of treatment duration on the risks of symptomatic recurrent VTE and clinically-relevant bleeding in children with first-episode, provoked, acute venous thrombosis differs between subgroups defined by type of sub-acute anticoagulant therapy in real-world clinical use (all prescribed clinically, with the exception of investigational dalteparin, which was prescribed under an investigator-held IND through December 2013).