Solexa Quality Score

The Solexa quality score is calculated differently from Phred Score. Instead of using the probability p, solexa quality score uses the odds p/(1-p).

Qsolexa = -10 log [p/(1-p)]
log is log base 10.
p = probability that the base calling is incorrect.

Solexa quality score is generated from the Illumina pipeline prior to version 1.3.