What kind of quality scoring scheme does Illumina use?
A quality score (or Q-score) expresses an error probability, and serves as a convenient and compact way to communicate very small error probabilities. Given an assertion, A, the probability that A is not true, P(~A), is expressed by a quality score, Q(A), according to the relationship:Q(A) =-10 log10(P(~A))where P(~A) is the estimated probability of an assertion A being wrong.A quality score of 10 means an error probability of 0.1, a quality score of 20 an error probability of 0.01, a quality score of 30 an error probability of 0.001,and so on.Quality scores are encoded into a compact form in FASTQ files which uses only one byte per quality value. As of CASAVA 1.8, in this encoding the quality score is represented as the character with an ASCII code equal to its value + 33.
