The
foundation of DDD is UniGene. UniGene employs a conservative method to assign
all the human EST sequences that meet minimal standards of quality to distinct
"clusters", each representing a unique human expressed gene. DDD
takes advantage of UniGene by comparing the number of times sequences from
different libraries were assigned to a particular UniGene cluster. This has
the advantage that DDD will only report on sequences that we have
confidence
There
will of course be many differences in the number of sequences contained in
each library that are assigned to a particular UniGene cluster, but only some
of these differences are likely to reflect biological reality. Therefore DDD
employs a statistical method of comparison - The Fisher Exact Test - to
identify only those differences that are likely to be real. One important
factor in determining statistical relevance is the absolute number of
sequences in each library that have been successfully assigned to a UniGene
cluster. In many cases there are not enough sequences
in
dbEST libraries to meet the threshold of significance employed in the Fisher
Exact Test. Since DDD will only yield a report if there are differences that
exceed this threshold, it is expected that many comparisons will yield
nothing.