I've taken two pre-employment polygraphs with two separate law enforcement agencies.  During the first screening, I was unaware of how the test works and what exactly it measures.  The tester ran through the standard procedure with me, asked me some questions and then we moved on to the actual testing phase.  I thought I was doing fine; I didn't have anything to hide and answered honestly.  After the test finished, he pointed out a couple questions he thought I had trouble with ( I know, now that I know how it all works, I cring to remember how niaive I was).  The first was whether there was anything in my past that might be used as blackmail against me while part of the department.  I hate to admit how boring my life really is, so I knew I couldn't have failed that question and said so.  The second question had to do with associating with people who used drugs.  I don't, but one of my parents was a lifelong addict and alcoholic, so the question may have stimulated a mere nervous response at the thought of my parent.  I explained this, but the polygrapher wasn't much interested; he pressed me on whether I currenlty knew people who used drugs.  I elaborated (I know, I know) that yes, there'd be situations where I'd be at a party or club and someone I knew on a passing basis would use something but that I was never part of it, nor did I condone it.  He asked me to write down my explanation and sign my name to it.    Needless to say, I faild that test.  I didn't even know why at the time because I'd been as honest as possible.

Then I found this sight and I finally understood why I inherently mistrusted the polygraph.  I read everything I could in order to understand the nature of the test and then it happened that I had to submit to another unrelated polygraph screening.  This time I was prepared.  I knew how the test worked, I knew what to look for I and I knew how to employ counter-measures to create the chart I wanted, not the one the tester was looking for.  It's not that I had anything to hide, but just like casinos and games of chance, the house advantage is significant with the the polygraph.  So I controlled my breathing, established a base rate and correctly responded to control and relevant questions.   

Given virtually the same test with the same questions, I passed the second test with flying colors.  Not because I was good at lying, but because I now know how the test works and how to control it myself.  Read up, get involved.  There probably should be a test that will screen out undesirable candidates, but this certainly isn't it.  If it were, would honest, innocent people be denied employment and corrupt, unscrupulous people be exposed in the media every day?