# Mapping Quality Scores

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Mapping Quality Scores quantify the probability that a read is misplaced. They were introduced by Heng Li and Richard Durbin in their paper describing MAQ and are usually reported on a Phred scale.

## Calculating a Mapping Quality Score

For a particular short sequence read, consider its best alignment in the genome. For this alignment, calculate the sum of base quality scores at mismatched bases and define a quantity SUM_BASE_Q(best). Also, consider all other possible alignments for the read. For the alignment i, define SUM_BASE_Q(i) as the sum of base quality scores at mismatched bases for that alignment.

Then, the mapping quality is defined as:

$MAPPING\_QUALITY = - log_{10} \left ({1.0 - \frac {10^{-SUM\_BASE\_Q(best)}} {\sum_i 10^{-SUM\_BASE\_Q(i)}}} \right )$

The quantity $10^{-SUM\_BASE\_Q(i)}$ tries to approximate the probability of generating a particular read when alignment i is used as template. For example, if there is a single mismatch with base quality 20, we approximate the probability of sampling the read as ~0.01; with two mismatches with base quality 20, the approximation becomes ~0.0001. Note that because this quantity will be effectively zero for most possible alignments, only a small subset of all possible alignments (those that result in small numbers of mismatches) must be considered in evaluating the denominator.

For paired end reads, we calculate SUM_BASE_Q as the sum of base quality scores at mismatched bases for both reads.

## Reference

Li H, Ruan J, Durbin R. (2008) Mapping short DNA sequencing reads and calling variants using mapping quality scores. Genome Research 18:1851-8.