Question

When processing high-throughput sequencing data, why does a Phred quality score of 30 represent a higher base-call accuracy than a Phred score of 20?

Accepted Answer

The Phred quality score is a logarithmic measurement used to estimate the probability that a specific base call made by a DNA sequencer is incorrect. The relationship between the Phred score and the probability of error is defined by the formula Q equals negative ten times the base-ten logarithm of the probability of error. At a Phred score of 20, the probability of an incorrect base call is 0.01, which corresponds to 99 percent accuracy. At a Phred score of 30, the probability of an incorrect base call decreases to 0.001, which corresponds to 99.9 percent accuracy. Because the scale is logarithmic, each increase of 10 in the Phred score represents a tenfold reduction in the error probability. Therefore, a Phred score of 30 is superior to a score of 20 because it represents an error rate that is ten times lower, leading to significantly higher confidence in the accuracy of the genetic sequence data.

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When processing high-throughput sequencing data, why does a Phred quality score of 30 represent a higher base-call accuracy than a Phred score of 20?

The Phred quality score is a logarithmic measurement used to estimate the probability that a specific base call made by a DNA sequencer is incorrect. The relationship between the Phred score and the probability of error is defined by the formula Q equals negative ten times....