Term Frequency
   Xxxxxxxxxxxxxx IBM xxxxxxxxxxx
xxxxxxxx xxxxxxxxxxx IBM xxxxxxx
xxxxxxxxxx xxxxxxxx Apple.  Xxxxxxxxxx
xxxxxxxxxx IBM xxxxxxxx.  Xxxxxxxxxx
xxxxxxxx Compaq.  Xxxxxxxxx xxxxxxx
IBM.
(Relative) term frequency can indicate
importance.
¡ Rd,f = fd,t
¡ Rd,t = 1 + ln fd,t
¡ Rd,t = (K + (1-K)           )