Why does explain plan show cost of 0

I am seeing the following in an explain plan..what gives?

(Cost=.00000000000000 Card=.00000000000000)

It runs ok, just curious

full plan follows.....

tks

steve

SELECT STATEMENT Optimizer=CHOOSE (Cost=2 Card=1 Bytes=193)   NESTED LOOPS (Cost=2 Card=1 Bytes=193)     PARTITION RANGE (ITERATOR)
      TABLE ACCESS (BY LOCAL INDEX ROWID) OF PS_PROJ_RESOURCE
(Cost=.00000000000000 Card=.00000000000000 Bytes=.00000000000000)

        INDEX (RANGE SCAN) OF PSAPROJ_RESOURCE (NON-UNIQUE)
(Cost=.00000000000000 Card=.00000000000000)

    INLIST ITERATOR
      TABLE ACCESS (BY INDEX ROWID) OF PS_PROJ_ACTIVITY (Cost=1 Card=19078 Bytes=305248)

        INDEX (UNIQUE SCAN) OF PS_PROJ_ACTIVITY (UNIQUE) (Cost=1 Card=19078) Received on Thu Jun 27 2002 - 11:24:23 CDT