The major components of the polygraph are a pneumographic bellow, a blood pressure cuff, electrodes, and a photoelectric plethysmograph. A defect in any of these components can affect the polygram. The subject's physical and mental condition can also produce effects unrelated to deception; for example, prescriptions and illegal drugs have an effect on some persons if taken prior to the test. The polygraphist's standards also warrant scrutiny. False positives are possible, because even a competent polygraphist will sometimes misinterpret reactions depicted on the polygram. Misinterpretations are possible because the polygraph technique includes standards for interpreting polygrams that are based in subjective decisions. Because of the possibility that a given polygraph test may be unreliable, before admitting polygraph evidence in a particular case, the trial judge should carefully scrutinize the circumstances surrounding the particular test, notably the sophistication of the instrument, the polygraphee's mental and physical conditions, and the polygraphist's credentials. Those circumstances have a critical effect on the trustworthiness of the particular diagnosis. A distortion in any of these three circumstances may warrant excluding the evidence in a particular case. A total of 19 footnotes are provided.