Phred Quality Score
Definition
A logarithmic estimate of base-calling error probability (Phred Q = -10 log10 P). Q20 = 1% error rate; Q30 = 0.1% error rate. Developed by Brent Ewing and Phil Green (University of Washington, 1998). Forensic sequencing guidelines require Q20 minimum; Q30 is preferred across the reported window.
- Formula
- Q = -10 log10(P)
- Q20
- 1% base-call error rate
- Q30
- 0.1% base-call error rate
- Origin
- Ewing and Green, University of Washington, 1998
- Forensic minimum
- Q20 required, Q30 preferred
Common questions
Why does forensic sequencing require a minimum Phred score rather than accepting any read?+
A low Phred score means a base call is more likely to be wrong, and an uncorrected miscall in a forensic sequence could be reported as a real genetic variant. Setting a Q20 floor, with Q30 preferred, filters out the least reliable base calls before results are interpreted.
Does a high Phred score guarantee the sequence itself is correct?+
No, it only reflects the confidence of each individual base call from the sequencing chemistry. Other errors, such as sample mix-up, contamination, or misalignment during analysis, are not captured by the Phred score and need separate quality controls.
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