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:
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.