What are Phred Scores (Q20)?
Phred Quality Scores Phred is a base-calling program for DNA sequence traces. The program was developed by Drs. Phil Green and Brent Ewing, and is copyrighted by the University of Washington. Phred’s base-specific quality scores are one of the most innovative features in Phred. After calling bases, Phred examines the peaks around each base call to assign a quality score to each base call. Quality scores range from 4 to about 60, with higher values corresponding to higher quality. The quality scores are logarithmically linked to error probabilities, as shown in the following table: Phred quality score Probability that the base is called wrong Accuracy of the base call 10 1 in 10 90% 20 1 in 100 99% 30 1 in 1,000 99.9% 40 1 in 10,000 99.99% 50 1 in 100,000 99.999% It has been shown that Phred’s error probabilities are very accurate – if Phred assigns a quality score of 40 to a base, the chances that this base is called incorrectly are indeed just 1 in 10,000. This high accuracy has been
