Answer to Question #119807 in Bioinformatics for Christopher

Question

Why E-value is dependent on the database and query size?

Answer 1

Why Is E-Value Dependent on The Database and Query Size?

The Expect value, also known as E-Value, is the measure of significance in a given alignment of a homology match. It is basically the possible random hits that can occur by chance when running a query sequence. E-Value is dependent on the size and length of the database. When running a query on a short identical database, the E-Value obtained will be much higher, and this may result in a false positive answer based on the number of hits such a scenario can be demonstrated by the use of short primer regions. When the default threshold, which is mostly 10 for the BLAST web page, is increased further, the resultant is the generation of more hits.