Door locks get graded based on a specific set of criteria that is accepted industry wide. The handle or lever of a lockset is graded using the same system, however the important part of the lockset is the deadbolt. The deadbolt is the part of your lockset that deters a break-in. There are minimum requirements that the deadbolts must meet to achieve the ranking
These grades were developed by Builders Hardware Manufacturers Association (BHMA) for the American National Standards Institute (ANSI). When shopping for locks you might see “ANSI Grade” or “BHMA Grade” or even “ANSI/BHMA Grade”. ANSI or BHMA use the same grading system.
For the most secure deadbolts you should, if possible, get a Grade 1 deadbolt. These are more expensive than Grade 2 or Grade 3 but with proper installation will provide the most benefit. Most deadbolts come with screws only long enough for the strike plate to be screwed in to the door frame, this is not a secure installation. The strike plate for the deadbolt should be screwed into the adjacent wood stud.
Most door frames are, at most, 1″ thick and easily damaged during a break-in attempt. However, the 2×4 that the door frame is attached to is much harder to damage and break because it is 1 1/2″ thick and attached to the 2×4 running horizontally at the top of the door frame. Your local home improvement stores will carry 3″ screws that can be used for this.