To v e r i f y th ' e r e l a t i o n s h i p s between the s t a t i s t i c a l models of word importance and t h s vector space model , dcsument c o l l e c t i o n s are used i n three d i f f e r e n t subjec t a reas , including aerodynamics ( cRAN ) , medicine ( MED ) and world a f f a i r s ( TIME ) .