Bioinformatics Slides (PDF) - Genomics & Medicine - Stanford ...

Statistical vs. Biological SignifcanceAssignmentFirst, for each search (MyHits, InterPro and BLAST hit), I would like you toreport some signifcance hits and describe why you think they aresignifcant both statistically and biologically; also report some statisticallyinsignifcant hits (and why) and are any of your statistically insignifcanthits, still signifcant biologically). To remind you what I said in class: astatistically signifcant fnd in the database search is always biologicallysignifcant, but a biologically signifcant result in the search is notnecessarily always statistically signifcant.Statistical signifcance and expectation values.Statistical signifcance is determined by the expectation value which gives youa measure of how likely this fnding is based on pure chance. A fndingwith an E-value of 1 or greater is not signifcant because it could occur bypure chance. A fnding with an E-value less than 10 -3 (one chance in athousand) is generally considered statistically signifcant (unless of courseyou are doing a 1,000 searches!). So the lower the expectation value, themore signifcant the fnding. Findings between 10 -3 and 1 are in the socalled twilight zone and require some further analysis or experiments todetermine their validity.