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Phred score (Q score)

Quality value for each sequenced base

The Phred score is a measure for base quality in DNA sequencing. The larger the Phred value, the better the quality of a sequenced base.

Phred   Error-rate    Accuracy
 10     1 of 10        90%       # very low quality
 20     1 of 100       99%       # minimum quality for many tools
 30     1 of 1000      99.9%     # reasonable good quality
 40     1 of 10000     99.99%    # high quality
 50     1 of 100000    99.999%   # very high quality


Quality Score Encoding

example quality string in fastq files (ASCII 33 Encoding)
%&?@AB

corresponding Phred quality scores
4 5 30 32 33






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