JavaScript must be enabled in order to use this site.Please enable JavaScript in your browser and refresh the page. Predicted quality (Q20) The number of called bases with a predicted quality of Q20 is reported. The predicted quality values are reported on the Phred scale, defined as -10 log10 (error probability). Q20, therefore, corresponds to a predicted error rate of one percent. Related Topics Parent topic: Quality following alignment Child TopicsPer-base quality score system Quality score predictors ×