The traditional polygraph Comparison Question Test (CQT) detects deceit in response to direct, accusatory questions, such as “Did you stab that man?” However, the Concealed Information Test (CIT), also known as the Guilty Knowledge Test, is a polygraph technique designed to detect a person’s guilty knowledge of a crime.
Illustration
A scenario illustrates how the CIT is used to support a homicide investigation. The body of a victim who apparently died of stab wounds is discovered inside a vacant house. Investigators analyze evidence from the crime scene to identify items most likely to be memorable and important to the perpetrator. The following objects are noted: a broken window used to gain entry; a bloody knife, identified as the murder weapon, near the body; a towel used to gag the victim; duct tape used to bind the victim; and a sleeping bag that covered the body. The investigating agency keeps this holdback information confidential.
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An identified suspect denies any knowledge of the crime. A CIT is constructed to determine if this individual recognizes elements from the crime scene that only the perpetrator would know. The investigator asks direct questions during the CIT.
1) Concerning the weapon used, was it a rope?
2) Was it a baseball bat?
3) Was it a handgun?
4) Was it a knife?
5) Was it a crowbar?
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It is unlikely that someone not involved with the crimewould consistently have strong reactions to the key items. |
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The polygraph examiner embeds the key question about the actual murder weapon—the knife—among several control questions about items not tied to the crime scene.[1]
Key Questions
During a CIT the examiner instructs the suspect to answer “no” to all questions on the test. The actual perpetrator of the crime usually has physiological reactions to the correct, or key, items that can be discriminated from those pertaining to the decoy or control items. It is unlikely that someone not involved with the crime would consistently have strong reactions to the key items. With the administration of numerous tests, it becomes almost mathematically impossible for an innocent person to react randomly to the majority of key questions.[2]
A numerical evaluation of the test data is conducted, resulting in a percentage of decision accuracy. The number of key items tested significantly impacts this precision; therefore, the CIT should include as many as possible. Conclusive results of a CIT with a large number of key items provide a high level of confidence.
The primary goal of key selection is to identify items that are significant and memorable to the perpetrator but unknown to an innocent examinee. A knife brought to a crime scene and used to stab a victim would be memorable to the perpetrator, while a coffee table broken during an altercation may not be. Items associated directly with the perpetrator’s objective in committing the crime make useful key items, while numbers or colors of items lack saliency.[3]
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The primary goal of key selection is to identify items that are significant and memorable to the perpetrator but unknown to an innocent examinee. |
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A rope used to bind a victim is more memorable to a perpetrator than the color of a victim’s clothing or the number of times that a victim was stabbed. Unexpected interruptions in the criminal’s plan also are significant. A valuable item stolen from the crime scene should be particularly relevant to the perpetrator. Key selection requires experience evaluating crime scenes from a behavioral perspective to ensure that items selected for testing have the highest potential for being memorable and salient to the offender.[4]
More information and additional articles can be found at http://leb.fbi.gov/2014/august?utm_campaign=email-Immediate&utm_medium=email&utm_source=fbi-law-enforcement-bulletin&utm_content=342090
