Answer to Question #119805 in Bioinformatics for Christopher

Question

Why BLAST hits with higher scores tend to have lower E-value?

Answer 1

The Expect value (E-value) represents the number of different alignments with scores that are identical to or better than the score that is predicted to occur during the search by chance. For example, an E value of 1 represents the situation when a database of the current size contains one match that has a similar score only by chance. As a result, lower E values reflect more significant score and quality (similarity) of alignment while higher scores indicate that the similarity is simply by chance.