Phred 

Phred base-calling is a computer program for identifying a base (nucleobase) sequence from a fluorescence trace" data generated by an automated DNA sequencer that uses electrophoresis and 4-fluorescent dye method. When originally developed, Phred produced significantly fewer errors in the data sets examined than other methods, averaging 40-50% fewer errors. Phred quality scores have become widely accepted to characterize the quality of DNA sequences, and can be used to compare the efficacy of different sequencing methods. Phred quality scores Q are defined as a property which is logarithmically related to the base-calling error probabilities. For example, if Phred assigns a quality score of 30 to a base, the chances that this base is called incorrectly are 1 in 1000. The most commonly used method is to count the bases with a quality score of 20 and above. The high accuracy of Phred quality scores make them an ideal tool to assess the quality of sequences. (wikipedia)