What's the credibility of polygraph test ?
I'm quoting from google search :
polygraph machine can detect lies. Commonly used in criminal investigations, this device actually measures nervous excitement. It operates on the premise that if a person is telling the truth they will remain calm.
But I've seen people telling lies calmly and to tell the truth they will get excited but it all depends on circumstances.
The accuracy (i.e., validity) of polygraph testing has long been controversial. An underlying problem is theoretical: There is no evidence that any pattern of physiological reactions is unique to deception. An honest person may be nervous when answering truthfully and a dishonest person may be non-anxious.