In these experiments the weighting functions involve three parameters
(
, 
and 
above). It was to be expected that their
values would interact, so there was a large number of preliminary runs
to try to determine the extent of interaction, and to give clues as to
the most fruitful ranges of values for further investigation. In should
also be noted that 
and 
are parameters in expressions
which are coefficients of a logarithmic value, whereas the document
length correction involving 
is not a logarithm; hence the value
of 
depends on the base of the logarithms used.
Table 1 compares two of the new models with baseline 
weighting (BM1 with 
), and with coordination level (BM0). For
the shorter queries (title + concepts) BM15 appears somewhat better than
BM1 (the difference is greater when a document length correction is
added---Table 3). But its performance on the long queries is
very poor. BM11 gives a very marked improvement over the baseline,
particularly for the long queries.
: Comparison of term weighting functions
