i am a bit confused how to read the current E value annotation on NCBI BLAST results.
I looked into the matter but could not find an factual answer .
What i understood 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*10^⁻19 (this would not result in negative values )
- or do they mean 4 E-19 as in : " four querys score better in S with - 19 difference"