N50L50

N50L50 is a term encountered in genome assembly evaluation to describe a contiguity measure that combines the N50 and L50 statistics. There is no universal definition, and usage varies by source. In many cases, N50L50 refers to reporting the two standard metrics together (often written as N50 and L50 or as N50/L50). In other contexts, it denotes a derived single value such as a ratio (N50/L50) or a product (N50×L50). Because there is no standard definition, readers should consult the original publication or tool documentation for the exact meaning.

Calculation basics: N50 is the length of the contig such that 50% of the assembly is contained in contigs of that length or longer; L50 is the number of contigs needed to reach 50% of the assembly length. If N50L50 is used as a ratio, it is computed as N50 divided by L50; if as a product, it is N50 times L50. When N50L50 is simply reported as a pair, it is interpreted as a summary alongside N50 and L50.

Interpretation and use: A higher ratio or product suggests greater contiguity per contig or greater aggregate contig length, but both definitions are sensitive to assembly size and length distribution. N50 and L50 remain more widely used because they are standard and intuitive. N50L50 should be considered in conjunction with other metrics and misassembly reports to gauge assembly quality.

Limitations: No single metric fully captures assembly accuracy. N50/L50-based measures can be biased by a few very long contigs or by fragmentation. Its use in benchmarking is limited; many pipelines report NGA50, misassembly rate, and completeness metrics in addition to N50 and L50.

See also: N50, L50, NGA50, NG50, QUAST, BUSCO.

---