Question: BLAST bit-score/score to E-value comparison for HSPs
1
gravatar for cklinger
20 months ago by
cklinger10
Canada
cklinger10 wrote:

For scores below a certain threshold (I believe it is something like 1e-179), BLAST reports the E-value simply as 0.0. I am interested in some cases in actually comparing the magnitude of E-values between hits, including those with E-values of 0.0 and so am trying to convert between either the score or bit-score, and the E-value.

My issue is that the comparison does not appear to yield the same values?

For example, take a Hsp with the following criteria:

  • bit-score: 320.857
  • score: 821
  • evalue: 9.0807e-104
  • eff_search_size: 17276649840
  • lambda: 0.267
  • kappa: 0.041

Comparison of the score and bit score works, suggesting the values of lambda and kappa are correct:

  • 0.267 * 821 - ln(0.041) / ln(2) = 320.857

However, no matter how I try to calculate the E-value, I get a different value:

  • 17276649840 * e^(-0.267 * 821 + ln(0.041)) = 4.465e-87
  • 17276649840 / 2^(320.857) = 4.465e-87

Considering I have seen these stated as the two methods to calculate an E-value from either the score or bit-score, I find it very confusing that these consistently give a different value than what BLAST reports.

Am I making a mistake in these calculations somewhere?

bit-score blast hsp e-value score • 1.2k views
ADD COMMENTlink written 20 months ago by cklinger10
Please log in to add an answer.

Help
Access

Use of this site constitutes acceptance of our User Agreement and Privacy Policy.
Powered by Biostar version 2.3.0
Traffic: 2251 users visited in the last hour