Polygraph vs Statement Analysis


Polygraphing as a means to determine between truth and fiction is a well-known and publicised method of Lie Detection. It involves connecting a subject to a variety of instruments in a specially prepared environment and subjecting the subject to a series of questions in order to obtain an acceptable reading from the polygraph.

The polygraph collects physiological data from at least three systems of the human body:

The respiratory system:

The examiner records thoracic breathing and abdominal breathing. Rubber tubes are placed over the chest and the abdomen in order to record these two aspects of breathing.

Galvanic skin response (electro dermal).

Two small metal sensors are connected to the fingers (non-connected fingers). These sensors record the filling of sweat glands and the nearly immediate reabsorption of the moisture.

The Cardiac system:

The examiner records Blood volume and Pulse rate. A blood pressure cuff is wrapped around the upper right arm, against the brachial artery. The cuff is connected via the instrument (cardiosphygmograph) to a hand pump with a pressure-indicating gauge.

Movement Detector

Certain systems (e.g. Lafayette) offer an option where the examiner can measure minor movements induced by the contraction of involuntary muscles used in order to attempt to confuse the polygraph readings.

These five (thoracic and abdominal breathing, blood volume, pulse rate and galvanic skin response) or six (including movement) readings are recorded on a chart. An examiner will do three to four charts per test o ensure consistent readings. From these charts, the examiner does his calculations (manually or using software), make his decision and produce a report, which goes to the client.

A Polygraph test may take up to one hour to complete per subject tested. The more information required, the longer the testing procedure.
Resistance is more regularly being encountered in the work-place against polygraphing which is treated with mistrust and experienced as being invasive because of all the instruments that need to be attached.

How reliable are polygraph tests then? One Magistrate's court (Booysens) once accepted (1996) two polygraph tests as evidence, but since then no other case has been reported. The CCMA has at times ruled in favour of the employer due to a polygraph test done, but has rejected the tests just as often. The polygraph test remains an investigative tool mainly and should be viewed primarily as that. The American Polygraph Association has determined that the polygraph is accurate to between 60 - 70%

Statement Analysis

Statement Analysis is believed to be as reliable as a polygraph examination. In this country, we must remember, the polygraph is an investigative tool, and not evidence. Since its introduction to the world during the 1920s, the polygraph has not succeeded in gaining admission in courts, except for rare exceptions.

Because Statement Analysis depends only upon the subject's statement, it's a "cold" technique. Statement Analysis scoring is based only upon statement structure and content, not upon behavioural responses such as gestures, perspiration, eye movements, and other factors that are open to individual interpretation. Thus Statement Analysis avoids the weakness of techniques that depend upon "global scoring." Statement Analysis is thus potentially more capable of development into a precise technique.

Statement Analysis is a versatile method because it is free from constraints that limit other techniques. It does not require attaching the subject to a machine as does a polygraph, and obtaining written permission. The subject is therefore unaware that his statement will be the object of specialised treatment and analysis.

Both the polygraph and the voice stress analysis method, require high-tech equipment, whereas in Statement Analysis only a pencil and paper is required. It also does not require the subject's presence during analysis.

Like other psychological and behavioural techniques, such as profiling, Statement Analysis serves to focus an investigation. It is a reasonably reliable method of probing for leads. This method is adaptable to many law enforcement investigative situations.