Don’t do them. They play games and are not accurate. Even the US Supreme Court has acknowledged that there is no consensus on polygraph reliability. Some courts have even held that the polygraph does not meet basic evidentiary standards enunciated in Daubert by the U.S. Supreme Court. Why is this? Especially when “everybody knows” that polygraphs are very useful in finding the guilty (at least on TV).
Polygraphs are good for telling if the person is stressed out. That’s all. So how can someone “fail” a polygraph? Easy. It is not the machine that determines whether a person passes or fails but the examiner. A common trick is to tell the examinee that s/he had failed. Then the examiner leaves the room. When they come back they ask the same questions again. If you change your answer, you lose. Some examiners use bad cop tactics and tell the examinee they KNOW you are lying. Since the U.S. Supreme Court has said that police can lie to you or trick you in trying to get you to confess, get what they do now? Yep. Lie and trick you. Play mind games with you.
Then they look for where the stress in your voice has been measured. Funny thing how “stress” and how much of it is “reasonable” is a subjective thing. And guess who decides? Not you.
Bottom line: DON’T DO POLYGRAPHS. You have nothing to gain and a great deal to lose.