Calculate what a Phred score means
Convert a Phred quality score into an approximate base-call error probability.
A FASTQ quality report shows a read tail with average Q20. What does Q20 mean in error-probability terms, and how should that shape a trimming discussion? Phred Q = -10 log10(P error) Treating Q scores as linear labels makes Q20 sound only a little worse than Q30, when the expected error probability is ten times higher. Start with the formula Q = -10 log10(P), so P = 10^(-Q/10). The formula converts a compact quality score back into an estimated probability of an incorrect base call. Substitute Q20 P = 10^(-20/10) = 10^-2. Dividing by 10 is the step that makes…
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