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
https://en.wikipedia.org/wiki/FASTQ_format
https://en.wikipedia.org/wiki/Phred_quality_score
https://www.illumina.com/documents/products/technotes/technote_Q-Scores.pdf