Two questions related to the topic

1. If I have an empty set of relevant results, then it would be
better
   to have no answers from the system at all. But neither precision
nor
   recall gives penalties for returning false positives in this case
   (0/1=0/2=...=0/100). How people handle with this ? Is there other
   measure for this cases ?

2. Let say I have ranking:

    1. A
    2. B *
    3. C
    4. D *
    5. F

   Where relevant answers are: B,D,E, and relevant answers found by
the
   system are marked with a star (*).

   Then for recall level 1/3 I have precision 1/2,
   for recall level 2/3 I have precision 2/4=1/2.

   The last position of ranking, false positive, is not counted in
the
   precision/recall measure, as in this measure "only positions where
an
   increase in recall is produced". I have the system which returns
some
   false positives at the end of ranking, but how can I measure/
compare
   it with other systems in terms of effectiveness, if precision/
recall
   does not take it into account ?

TIA,
Maciej