# How to give entire word a quality score to account for the phred score of each letter?

I have a word representing a DNA sequence and each letter of that word is a nucleotide base (A,T,G,C). Each letter has a phred quality score. The formula used for the phred quality score is $Q=-10\log_{10} P$ where $Q$ is the phred score and $P$ the probability of that letter to be incorrect( for example, there was a problem in DNA sequencing).

My question is:

what is the best way to give the entire word a quality score, so that it takes into account the phred score of each letter?

One thing I though about was to take the average and another was to add the phred scores.

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 I think a good tag for this post would be phred, but I cannot create a tag. – Julio Diaz Jun 18 '12 at 18:55