Figure 1 Semi-Markov probability model

After the treatment decision node (square node), the cohort is split into groups based on Expanded Disability Status Scale (EDSS) states. During each 6-month cycle, subjects may die, develop progressive multifocal leukoencephalopathy (PML) (if taking natalizumab), have a relapse and progress in disability, or have a relapse without disability progression. All subtrees ending with [+] are identical to the subtree above; the interferon (IFN) β-1a arm is identical to the natalizumab arm, except that the probability of developing PML is zero. MS = multiple sclerosis.