BOL

A quick guide to Phred scaling

• Phred value = −10 * log10(ε)

• Examples:
• 90% confidence (10% error rate) = Q10
• 99% confidence (1% error rate) = Q20
• 99.9% confidence (.1% error rate) = Q30

• SAM encoding adds 33 to the value (because
ASCII 33 is the first visible character)