Phred scale

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A scale commonly used to measure data quality and error rates in sequence analysis. It was popularized by Phil Green's PHRED base-calling software.

It is defined as:


  PhredScore = -10 log10 (\epsilon)

Thus, a base call that is expected to be correct 99% of the time (1% error rate), would be assigned a phredScore = 20. A base call expected to be accurate 99.9% of the time (0.1% error rate), would be assigned a phredScore = 30.