**E-value**

**E-value**

**The BLAST E-value is the number of expected hits of similar quality (score) that could be found just by chance.**

E-value of 10 means that up to 10 hits can be expected to be found just by chance, given the same size of a random database.

E-value can be used as a first quality filter for the BLAST search result, to obtain only results equal to or better than the number given by the -evalue option. Blast results are sorted by E-value by default (best hit in first line).

**blastn -query genes.ffn -subject genome.fna ****-evalue 1e-10**

**The smaller the E-value, the better the match.**

-evalue **1e-50**

*small E-value: low number of hits, but of high quality*

Blast hits with an E-value smaller than 1e^{-50} includes database matches of very high quality.

-evalue **0.01**

Blast hits with E-value smaller than 0.01 can still be considered as good hit for homology matches.

-evalue **10** (default)

*large E-value: many hits, partly of low quality*

E-value smaller than 10 will include hits that cannot be considered as significant, but may give an idea of potential relations.

The E-value (expectation value) is a corrected bit-score adjusted to the sequence database size.* *The E-value therefore depends on the size of the used sequence database. Since large databases increase the chance of false positive hits, the E-value corrects for the higher chance. It's a correction for multiple comparisons. This means that a sequence hit would get a better E-value when present in a smaller database.

** ****E** = **m** x **n** **/ 2**^{bit-score}

m - query sequence length

n - total database length (sum of all sequences)

**Bit-score**

**Bit-score**

**The higher the bit-score, the better the sequence similarity**

The bit-score is the requires size of a sequence database in which the current match could be found just by chance. The bit-score is a log_{2} scaled and normalized raw-score. Each increase by one doubles the required database size (2^{bit-score}).

Bit-score does not depend on database size. The bit-score gives the same value for hits in databases of different sizes and hence can be used for searching in an constantly increasing database.

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