Which output from a BLAST search provides an estimate of the number of false positives from a BLAST search? (a) E value; (b) bit score; (c) percent identity; or (d) percent positives.
Question
Which output from a BLAST search provides an estimate of the number of false positives from a BLAST search? (a) E value; (b) bit score; (c) percent identity; or (d) percent positives.
Solution
The output from a BLAST search that provides an estimate of the number of false positives from a BLAST search is (a) E value. The E value, or expect value, is a parameter that describes the number of hits one can "expect" to see by chance when searching a database of a particular size. It decreases exponentially as the Score (S) of the match increases. Essentially, the E value describes the random background noise. An E value of 1 assigned to a hit can be interpreted as meaning that in a database of the current size one might expect to see 1 match with a similar score simply by chance.
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