The polygraph instrument is a scientific instrument that is designed to sense and record the physiological changes in the human body during questioning. The following are the objectives of a polygraph examination:
- Obtain additional investigation leads to the facts of the case/offenses
- Ascertain whether the subject is telling the truth
- Verify and compare conflicting statement
- Locate the fruits or tools of the crime and whereabouts of wanted persons
- Identify other persons involved (e.g. accomplices)
- Obtain valuable information from reluctant witnesses
- Eliminate innocent suspects
In order to achieve these objectives, the polygraph must be subject to the rigorous testing of experts in the field so that it can become an acceptable scientific instrument.
In research, an object is considered reliable if its results are reproducible if used on a different occasion or by a different person. That is, you can expect it to perform consistently well.
An object is valid if it is logically or factually sound. Validity is the rating of how accurate an object measures what it is supposed to measure.
In the context of the polygraph instrument, if we are inquiring on its reliability and validity, it means that we want to confirm whether it can be used by various people at various times and still produce trustworthy results, and whether it measures the body’s physiological changes correctly.
Or, in other words, we want to know whether polygraph examiners can consistently detect deception using the polygraph instrument.
Understandably, it is difficult to conduct objective research into the polygraph instrument due to the nature of how it attempts to detect deception.
Unlike ballistics research where it is easy to replicate fired bullets, it is markedly more difficult to simulate the psycho-physiological responses of a criminal suspect in a laboratory setting or in a mock crime situation. Doing it in a lab could diminish the actually body responses even if actual criminals are used in the study. When using non-criminals in a simulation, they would not have the same levels of fear and motivation as suspects would. Additionally, examination results from a polygraph examiner are generally not subject to full confirmation.
Various research today still show that the polygraph is not a valid tool for use in the criminal justice system or that its validity is immeasurable.