我在分析xiaoRNA的数据，但是Read depth是什么？为什么要算？

Read depth是 短读mapping后的密度，是二代测序的重要信号。RNA-seq也得算密度吧？

More information in detail:

MEAN (MAPPED) READ DEPTH(Given by illumina)
The mean mapped read depth (or mean read depth) is the sum of the mapped read depths at each reference base position, divided by the number of known bases in the reference. The mean read depth metric indicates how many reads, on average, are likely to be aligned at a given reference base position.
http://www.illumina.com/truseq/q ... e_distribution.ilmn