Sequencing read

A sequencing read is an ordered series of base calls inferred from one sequencing measurement or molecule-associated signal. It is evidence about a nucleotide sequence, rather than the original molecule itself. A read can also carry base-quality estimates expressing uncertainty in individual calls. ^[ref]

For a base-call error probability $p$, a Phred quality score is $Q=-10{\log }_{10}p$: $Q=20$ corresponds to an estimated error probability of 0.01. A long read with uncertain bases and many short overlapping reads therefore provide different kinds of evidence. Aligning reads, building a consensus and interpreting a mutation are further inference steps. Repeated reads need not represent independent starting molecules, especially after amplification.