Question

When using BLAST, why does a lower E-value indicate a more statistically significant match compared to a higher E-value?

Accepted Answer

The E-value, or Expect value, represents the number of hits one can expect to see by chance when searching a database of a particular size. BLAST works by aligning a query sequence against a database of sequences to find regions of similarity. Because sequences can sometimes appear similar simply due to random arrangements of amino acids or nucleotides, BLAST calculates the probability that a given alignment occurred by pure coincidence. A lower E-value means that it is highly unlikely the match happened by random chance, indicating that the similarity is biologically meaningful and likely due to shared evolutionary ancestry. For example, an E-value of 0.001 means there is a 1 in 1000 chance that this match would occur randomly in a database of that size. A higher E-value, such as 10, suggests that you would expect to see 10 similar matches just by luck, which makes the alignment statistically insignificant and unreliable. Therefore, as the E-value approaches zero, the confidence that the match represents a true biological relationship increases, making a lower value a stronger indicator of a significant match.

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When using BLAST, why does a lower E-value indicate a more statistically significant match compared to a higher E-value?