*R*^{a,b} values for synthetic datasets T.

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We considered simulated datasets consisting of 100 outbreak sizes; the vertical arrows show the position of the 50^{th} tree. Panel (a) corresponds to *R* = 0.6 and shows 2ln*B*(T_{a}) as a function of information in T_{a}, as T_{a} increases from including the first tree only to including the first *N*−1 trees. The maximum of 2ln*B*(T_{a}) is below 10, suggesting preference for a single *R* estimate over the whole dataset. Panel (b) is similar to panel (a), except T consisted of 50 trees with *R*^{a} = 0.6 and 50 trees with *R*^{b} = 0.85. Values of 2ln*B*(T_{a}) larger than 10 show where the dataset T can be split into T_{a,b} with justification for evaluating *R*^{a,b}. The corresponding *R*^{a,b} values are shown in the panel (c) as a function of 2ln*B*(T_{a}).