Table 3. Definition of different term-weighting approaches.

Note that some formulations include the expression $\mathrm{m}\mathrm{a}\mathrm{x}(X,1)$ to prevent the possibility of undefined values, such as divisions by zero or $\log (0)$.

Scheme	Formulation
TGF (Salton & Buckley, 1988)	A + C
IDF (Salton & Buckley, 1988)	$\log (N/(A+C))$
TGF* (Domeniconi et al., 2015)	A
TGF*-IDFEC (Domeniconi et al., 2015)	$A\times (\log ((C+D)/\mathrm{m}\mathrm{a}\mathrm{x}(C,1)))$
$\chi {}^2$ (Quinlan, 1986)	$N((AD-BC{)}^2/((A+C)(B+D)(A+B)(C+D)))$
OR (van Rijsbergen, Harper & Porter, 1981)	$\log ((\mathrm{m}\mathrm{a}\mathrm{x}(A,1)\times D)/\mathrm{m}\mathrm{a}\mathrm{x}(B\times C,1))$
IG (Quinlan, 1986)	$(A/N)\log (\mathrm{m}\mathrm{a}\mathrm{x}(A,1)/(A+C))-((A+B)/N)\log ((A+B)/N)+(B/N)\log (B/(B+D))$
GR (Quinlan, 1986)	$IG/(-((A+B)/N)\log ((A+B)/N)-((C+D)/N)\log ((C+D)/N))$
FDD ${}_{0.5}$ (Maisonnave et al., 2021)	$(1.25\times A/(A+C)\times A/(A+B))/((0.25\times A/(A+C))+A/(A+B))$
FD $D_{1.0}$ (Maisonnave et al., 2021)	$(2.00\times A/(A+C)\times A/(A+B))/((1.00\times A/(A+C))+A/(A+B))$
FDD ${}_{10}$ (Maisonnave et al., 2021)	$(101\times A/(A+C)\times A/(A+B))/((100\times A/(A+C))+A/(A+B))$