This is a test version of Biostars. For the public version, visit https://www.biostars.org.
The Meaning Of The E In Blast Output

I have a question that's been floating around in my head for a while, and a colleague recently asked the same thing so I thought I should finally get to the bottom of it.

What does the 'e' in the output of a BLAST search refer to? Specifically, does the symbol 'e' refer to 'x10^' or does it refer to the mathematical constant 'e=2.72...'? I could see a justification for either: e as 'x10^' is a convenient notation, while 'e=2.72...' is actually used in the calculation of the e-value score itself.

This hasn't really mattered in assessing the relative quality of a hit, or determining the homology between sequences, but I thought I'd better find an answer for this question.

Thanks!

David

blast
• 3,437 views • link
updated by Philipp Bayer Philipp Bayer • written by David M David M

2 answers

The "e" stands for "10^".

This example output has an e-value of "5.0e-120".

In this context, it would make no sense for e to be the Euler number, as that would mean: 5 * 2.72 * (-120), which is -1632, but e-values can't be smaller than 0!

(Here's another answer for the same question on researchgate)

• 14 views • link
written by Philipp Bayer Philipp Bayer

Thanks for the answer! When I mentioned "e as 2.72...", I was actually referring to 5.0e^(-120), using your example.

• 0 views • link
written by David M David M

It's the probability that there is a hit in the database having better match (alignment score) to your query.

• 35 views • link
written by Vitis Vitis

Sorry, but not only does this not answer the question, it's wrong. The E-value is not a probability at all, which you can verify when you see it taking values greater than 1.

See the documentation: "The Expect value (E) is a parameter that describes the number of hits one can "expect" to see by chance when searching a database of a particular size."

As the poster below states, the e notation is "10^" because it's the only thing that makes sense.

• 0 views • link
updated by Ram Ram • written by matted matted

Log in to answer this question.

More posts like this