My opinion is that, ultimately, the polygraph is there to ensure that the applicant is truthful though intimidation.  This theory suggests that if an applicant knows he'll face a "lie-detector", he'll either be truthful on the application, or (if he's had a little more criminality in his background) he'll not apply at all.  In either case, the intimidation works to the favor of the hiring department.

Hence, to some departments, the polygraph may be little more than a formality, with the understanding that the applicant has been sufficiently intimidated into telling the truth throughout the process.  Other departments may rely on it for actual "fact finding" (i.e. F.B.I. et al).

I suspect that overall, most people (>50%) who receive pre-employment polygraph interrogations ulitmately "pass." However, the failure rate varies widely depending on the agency involved. For example, in both the FBI and the Los Angeles Police Department, the failure rate is on the order of 50%. But in the Department of Defense (DoD) counterintelligence-scope polygraph screening program, everyone who does not make "substantive admissions" seems to pass (according to DoD's last two annual reports to Congress on the polygraph program).

I think that your suggestion, "If you go in and admit to something small (like smoking MJ) and then take the poly and stick to your story, almost always you pass" is ill-founded. While this may hold true for certain polygraph programs (like the DoD one), in other agencies, many truthful persons who make no "post-test" admissions nonetheless fail. See the narratives on the AntiPolygraph.org Personal Statements page for examples.

Is it just me or do the majority of people who take a pre-employment poly for law enforcement or gov't jobs pass? I've noticed that almost all those who are applying for LE and gov't pass except those who admit to something shady or bad such as using hard drugs or criminal acts. Those people always get a hard time and usually fail the poly and get booted from the process. If you go in and admit to something small(like smoking MJ) and then take the poly and stick to your story, almost always you pass. Any opinions?