Quality Scores
A quality score (or Q-score) expresses an error probability. In particular, it serves as a convenient and compact way to communicate very small error probabilities.
Given an assertion, A, the quality score, Q(A), expresses the probability that A is not true, P(~A), according to the relationship:
Q(A) =-10 log10(P(~A))
where P(~A) is the estimated probability of an assertion A being wrong.
The relationship between the quality score and error probability is demonstrated with the following table:
|
Quality score, Q(A) |
Error probability, P(~A) |
|---|---|
|
10 |
0.1 |
|
20 |
0.01 |
|
30 |
0.001 |
Note
On supported Illumina systems, Q-scores are automatically binned. The specific binning applied depends on the current Q-table. See the white paper Reducing Whole Genome Data Storage Footprint for more information, available from the Illumina website.