Table A.3.

Overview of Phred quality scores and the corresponding base calling accuracies. The table shows different Phred scores, the probability of an incorrect base call as well as the respective accuracy per base for the appropriate quality scores. Q30, i.e. 99.9% accuracy or 1 error per 1000 bases, is generally considered to be a benchmark for quality (http://www.illumina.com/documents/products/technotes/technote_Q-Scores.pdf).

Phred quality score	Probability of incorrect base call	Base call accuracy
10	1 in 10	90%
20	1 in 100	99%
30	1 in 1000	99.9%
40	1 in 10,000	99.99%
50	1 in 100,000	99.999%