Relative apparent synapomorphy analysis, or RASA, is a method that aims to determine whether a given character is shared between taxa

Taxon

|thumb|270px|[[African elephants]] form a widely-accepted taxon, the [[genus]] LoxodontaA taxon is a group of organisms, which a taxonomist adjudges to be a unit. Usually a taxon is given a name and a rank, although neither is a requirement...

 due to shared ancestry or due to convergence. A synapomorphy

Synapomorphy

In cladistics, a synapomorphy or synapomorphic character is a trait that is shared by two or more taxa and their most recent common ancestor, whose ancestor in turn does not possess the trait. A synapomorphy is thus an apomorphy visible in multiple taxa, where the trait in question originates in...

 is a shared trait found among two or more taxa and their most recent common ancestor, whose ancestor in turn does not possess the trait. RASA assigns a score to the character based on its potential to be informative.

Limitations

The method performs poorly when used to select an outgroup taxon, to quantify the amount of phylogenetic signal

Phylogenetics

In biology, phylogenetics is the study of evolutionary relatedness among groups of organisms , which is discovered through molecular sequencing data and morphological data matrices...

 present, or to identify taxa that may be prone to long branch attraction

Long branch attraction

Long branch attraction is a phenomenon in phylogenetic analyses when rapidly evolving lineages are inferred to be closely related, regardless of their true evolutionary relationships. For example, in DNA sequence-based analyses, the problem arises when sequences from two lineages evolve rapidly...

.