= 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. <A weakly structured stem for human origins in Africa by Aaron P. Ragsdale and others>[2023, population-genetic inference and frequency-spectrum modelling]{ref}
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