Searches were carried out automatically by means of City University's
Okapi text retrieval software. The weighting functions described in
Sections 4--6 were implemented as BM15
(the model using equation 8 for the document term
frequency component) and BM11 (using equation 10). Both functions
incorporated the document length correction factor of equation 13.
These were compared with BM1 (
weights, approximately ICF,
since no relevance information was used in these experiments) and
with a simple coordination-level model BM0 in which terms are given
equal weights. Note that BM11 and BM15 both reduce to BM1 when 
and 
are zero. The within-query term frequency component
(equation 15) could be used with any of these functions.
To summarize, the following functions were used:
In the experiments reported below where 
is given as 
,
the factor 
is implemented as 
on its own (equation 16).