 Researchers must decide which character states are "ancestral" ("plesiomorphies") and which are derived ("synapomorphies"), because only synapomorphic character states provide evidence of grouping.This determination is usually done by comparison to the character states of one or more "outgroups".States shared between the outgroup and some members of the in-group are symplesiomorphies; states that are present only in a subset of the in-group are synapomorphies.