Body: <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>
  
Comments.ContentLicense: CC BY-SA 4.0
Comments.CreationDate: 2021-02-09T18:38:35.207Z
Comments.Text: It's a number in regular scientific format, so it's the former. `4e-19` = `4 * 10^-19`
Comments.UserDisplayName: Ram RS
Comments.ContentLicense: CC BY-SA 4.0
Comments.CreationDate: 2021-02-09T19:16:11.697Z
Comments.Text: 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
Comments.UserDisplayName: user12256545
Comments.ContentLicense: CC BY-SA 4.0
Comments.CreationDate: 2021-02-09T19:18:40.313Z
Comments.Text: After rereading i could have seen earlier that he is apparently wrong.
Comments.UserDisplayName: user12256545
Comments.ContentLicense: CC BY-SA 4.0
Comments.CreationDate: 2021-02-10T22:16:19.970Z
Comments.Text: He never says anything that resembles your second interpretation. How did you end up with that interpretation?
Comments.UserDisplayName: Ram RS
Comments.ContentLicense: CC BY-SA 4.0
Comments.CreationDate: 2021-02-13T12:17:18.773Z
Comments.Text: 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.
Comments.UserDisplayName: user12256545
Comments.ContentLicense: CC BY-SA 4.0
Comments.CreationDate: 2021-02-13T14:24:19.977Z
Comments.Text: 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.
Comments.UserDisplayName: Ram RS
Tags: <phylogenetics><r><blast>
Title: How to read E-value annotation on NCBI BLAST?
ThreadText: # 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.  
  
  ---
  
  ## 0 answers
  
  