||05-06-2011 11:42 PM
Properly installed entry locksets can't be opened with a credit card. That is the sole purpose of the small secondary plunger right behind the latch bolt. The larger latch bolt drops into the hole in the strike plate and the small secondary plunger does not, this locks the latch bolt in the extended position so that it can not be slipped with a credit card.
If you are trying to further secure an interior door, then you want to install an entry lockset. It must be installed so the strike plate is tight enough that the small plunger can NOT drop into the strike plate hole, if it is possible to shut the door hard enough to get the secondary plunger to drop into the strike hole then the latch bolt is no longer secure from being slipped open. Interior locksets do not have the secondary plunger.
Of course if someone really wants in that room an interior door is no match for a prybar or even a swift kick, a deadbolt won't help that situation much either though.