Count if it contains string="1 Week"

Hi Guys,

I need help to count the strings value that it how many times occurs in fields. I tried with using variable but no luck formate of my vairable is 

$F{Weeks}=="1 Week"  and calculation is set to count/sum but it always return 0 value to variable.

Help is appreciated !

Thanks In advance  :)

Thanks 

Pawan