Unfolded site frequency spectrum

The unfolded site frequency spectrum counts segregating sites according to the number of sampled copies carrying the derived allele. In a sample of five haploid sequences, a mutation carried by one sequence contributes to the singleton class, while one carried by four contributes to class four. Assigning ancestral and derived states is essential: reversing them exchanges classes $i$ and $n-i$. A folded site frequency spectrum removes this directional distinction. ^[ref]