<p>I am seeking clarification on how to read the  <code>E</code> value annotation on NCBI's BLAST results.</p>
<p>What I understand so far:</p>
<blockquote>
<p>E -values are the expected value of the number of random querys of the same length as the input that would result in a even or better S-score on the same sized database. Since E-values are estimates, not probabilities , they can be lower than 0.</p>
</blockquote>
<p>However, if NCBI BLAST retuns an e-value of say: &quot;4e -19&quot; do they mean:</p>
<ul>
<li><span class="math-container">$ {4 \cdot 10^{-19}} $</span> (this would not result in negative values )</li>
<li>or do they mean 4 E-19 as in : &quot;<em>four queries score better in S with a -19 difference</em>&quot;</li>
</ul>

CC BY-SA 4.0
2021-02-09T18:38:35.207Z
It's a number in regular scientific format, so it's the former. `4e-19` = `4 * 10^-19`
Ram RS
CC BY-SA 4.0
2021-02-09T19:16:11.697Z
thanks for the reassurance! .. man, sometimes i stumble upon one source that uses a weird notation and i start to get quite confused.. The source of my confusion was Victor Flores comment on  https://www.researchgate.net/post/What_is_E-value_exactly_means_and_what_is_it_1e-63_representing
user12256545
CC BY-SA 4.0
2021-02-09T19:18:40.313Z
After rereading i could have seen earlier that he is apparently wrong.
user12256545
CC BY-SA 4.0
2021-02-10T22:16:19.970Z
He never says anything that resembles your second interpretation. How did you end up with that interpretation?
Ram RS
CC BY-SA 4.0
2021-02-13T12:17:18.773Z
You are totally right, i confused "beyond" with "below" (not a native english speaker) . This should never have resulted in such a stupid confusion,  unfortunately the feedback at university is a little bit sparse due to the pandemic circumstances , so you had to bare with my undergrad incertitude. I honestly thank you for clearing it out nonetheless. On the upside i learned my lesson, took a step back and studied Altschuls formulation.
user12256545
CC BY-SA 4.0
2021-02-13T14:24:19.977Z
Don't worry about it, glad I could help. A lot of the long-winded description of these terms can make one confused but as you gain some practical experience, you'd understand these better.
Ram RS
<phylogenetics><r><blast>
How to read E-value annotation on NCBI BLAST?
# How to read E-value annotation on NCBI BLAST?

(Asked by: username_0 on 2021-02-09)

I am seeking clarification on how to read the `E` value annotation on NCBI's BLAST results.

What I understand so far:

> E -values are the expected value of the number of random querys of the same length as the input that would result in a even or better S-score on the same sized database. Since E-values are estimates, not probabilities , they can be lower than 0.

However, if NCBI BLAST retuns an e-value of say: "4e -19" do they mean:

* $ {4 \cdot 10^{-19}} $ (this would not result in negative values )
* or do they mean 4 E-19 as in : "*four queries score better in S with a -19 difference*"

**Tags:** phylogenetics, r, blast
> **Comments:**
> **username_1**: It's a number in regular scientific format, so it's the former. `4e-19` = `4 * 10^-19`  
> **username_0**: thanks for the reassurance! .. man, sometimes i stumble upon one source that uses a weird notation and i start to get quite confused.. The source of my confusion was Victor Flores comment on  https://www.researchgate.net/post/What_is_E-value_exactly_means_and_what_is_it_1e-63_representing  
> **username_0**: After rereading i could have seen earlier that he is apparently wrong.  
> **username_1**: He never says anything that resembles your second interpretation. How did you end up with that interpretation?  
> **username_0**: You are totally right, i confused "beyond" with "below" (not a native english speaker) . This should never have resulted in such a stupid confusion,  unfortunately the feedback at university is a little bit sparse due to the pandemic circumstances , so you had to bare with my undergrad incertitude. I honestly thank you for clearing it out nonetheless. On the upside i learned my lesson, took a step back and studied Altschuls formulation.  
> **username_1**: Don't worry about it, glad I could help. A lot of the long-winded description of these terms can make one confused but as you gain some practical experience, you'd understand these better.  

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## 0 answers

