def getRelativeQualityScore(self, i, score_type="<STR_LIT>"):
<EOL>if self.seq_qual is None:<EOL><INDENT>raise NGSReadError("<STR_LIT>" +<EOL>"<STR_LIT>" + str(i) + "<STR_LIT>" + self.name +<EOL>"<STR_LIT>")<EOL><DEDENT>if i < <NUM_LIT:0> or i > self.seq_qual:<EOL><INDENT>raise NGSReadError("<STR_LIT>" +<EOL>"<STR_LIT>" + str(i) + "<STR_LIT>" + self.name +<EOL>"<STR_LIT>" +<EOL>"<STR_LIT>" + str(len(self.seq_qual)))<EOL><DEDENT>val = self.seq_qual[i]<EOL>if score_type == "<STR_LIT>":<EOL><INDENT>return ord(val) - self.LOWSET_SCORE_ILL_18_PHRD_33<EOL><DEDENT>elif score_type == "<STR_LIT>":<EOL><INDENT>return ord(val) - self.LOWSET_SCORE<EOL><DEDENT>else:<EOL><INDENT>raise NGSReadError("<STR_LIT>" + score_type)<EOL><DEDENT>
Get the realtive quality score (i.e. the phred quality score) for a given
base.

:raise: NGSReadError if the index is less than 0 or more than l-1 where l
        is the length of the read; NGSReadError if the encoded quality
        score at the given location is outside the expected range;
        NGSReadError if the read has no quality string; NGSReadError if
        sepficied encoding scheme is unknown.