Because PICRUSt, and evolutionary comparative genomics in general, depends on sequenced genomes, biological samples from well-studied environments (many sequenced genomes) will be better predicted than poorly studied environments. In order to assess how many genomes are available, PICRUSt optionally allows users to calculate a Nearest Sequenced Taxon Index (NSTI) for their samples. This index reflects the average phylogenetic distance between each 16S rRNA gene sequence in their sample, and a 16S rRNA gene sequence from a fully sequenced genome. In general, the lower the NSTI score, the more accurate PICRUSt's predictions are expected to be. For example, showed that PICRUSt was much more accurate on diverse soil samples and samples from the Human Microbiome Project than on microbial mat samples from Guerrero Negro, which contained many bacteria without any sequenced relatives.
