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

These quality symbols in fastq files (ASCII 33 Encoding)

% & ? @ A

correspond to Phred quality scores:

4 5 30 31 32