Why Default blast search result shows fractional scores at last?

Question

[barathvajramachandran]

I've run Blast normally with default parameters. Alignment scores are give in fractions like 84.3. But the BLOSUM 62 matrix contains no fractional scores for any amino acids. So I would like to know how those fractional score is arrived. http://www.ncbi.nlm.nih.gov/protein/9501247?report=fasta

http://www.ncbi.nlm.nih.gov/Class/FieldGuide/BLOSUM62.txt

Answer 1

[pld]

http://homepages.ulb.ac.be/~dgonze/TEACHING/stat_scores.pdf

http://www.ncbi.nlm.nih.gov/blast/tutorial/Altschul-1.html

You have to remember that BLAST is not a simple sequence aligner, it tries to find you the likely best hit(s) as quickly as possible. This means BLAST trades accuracy (not precision) for speed, which means it uses many heuristics to speed the search of the sequence space up.

The score isn't an actual alignment score, but a transformation on that score to normalize on the size of the search space and the scoring system used.

If you just want a raw score, you shouldn't be using BLAST.