A piecewise constant coalescent Bayesian skyline prior was used with ten groups. All models were run until the effective sample size (ESS) for the root height was greater than 1000. The tMRCA is reported as the geometric mean of the posterior distribution (Morrison, 2008). The population parameter returned by the skyline plot is Neτ, where Ne is the effective population and τ is the generation length in years. Because τ is constant, the use of Neτ allows evaluation of relative changes in the effective population of HCV.