where nk, d is the number of occurrences of keyphase k in
document d. Parameter c∈ [0, 1] is the predefined
minimum tf value for every keyphrases. The value of c is
chosen through experimenting. This modification make
sure no keyphrase will received a too small tf, which make
sense since in our previous keyphrase was extraction in
supervised manner. The new formula also possess more
flexibility, the value of parameter c can be tuned to suit
specific application.