# N50

### From ArachneWiki

**N50** is a statistical measure of average length of a set of sequences. It is used widely in genomics, especially in reference to contig or supercontig lengths within a draft assembly.

Given a set of sequences of varying lengths, the N50 length is defined as the length *N* for which 50% of all bases in the sequences are in a sequence of length *L < N*. This can be found mathematically as follows: Take a list *L* of positive integers. Create another list *L' *, which is identical to *L*, except that every element *n* in *L* has been replaced with *n* copies of itself. Then the median of *L' * is the N50 of *L*. For example: If *L* = {2, 2, 2, 3, 3, 4, 8, 8}, then *L' * consists of six 2's, six 3's, four 4's, and sixteen 8's; the N50 of *L* is the median of *L' *, which is 6.