A sequencing run generates reads that sample a genome randomly and independently . These reads are not distributed equally across an entire genome; some bases are covered by fewer reads, some by more reads than the average coverage. Coverage refers to the average number of times a single base is read during a sequencing run. If the coverage is 100 X, this means that on average each base was sequenced 100 times. The more frequently a base is sequenced, the more reliable a base is called, resulting in better quality of your data. The Lander / Waterman equation is one method for determining coverage. C=LN/G, where C is coverage, L is read length, N is the number of reads and G is the haploid genome length.