The Comparison Question Test: Does It Work and If So How?

polygraphHeinz and Suzanne Offe have just published a paper in Law and Human Behavior, in which they present the results of a study exploring when and how the controversial Control Question Test works in polygraph testing.

The logic of the CQT is that innocent subjects will respond more strongly to Control Questions (CQs, which relate to previous history of – or inclination towards – wrong-doing) than to Relevant Questions (RQs, which relate to the particular offence being investigated). Guilty subjects, on the other hand, will, it is theorised, respond more strongly to RQs.

In order for this procedure to be effective, it is claimed, subjects need to be convinced that being judged ‘not guilty’ depends on them giving socially desirable responses to the CQs. Examiners will tell their subjects something along the following lines:

“I want to find out whether you are the sort of person capable of [the crime under investigation] based on your history. So the questions I am going to ask you about your history will allow me to make these judgements about you. Now, tell me if you have ever taken something that was not yours…”.

In reality the explanations are a lot more detailed than this, all designed to raise the anxiety an innocent subject might feel at the prospect of being accused of something they did not do. (Offe and Offe give a detailed example of how this is done in the first appendix to their study.)

However, as Offe and Offe point out, it is debatable whether or not this type of questioning actually does increase the salience of the CQs for subjects.

The researchers set out to test the workings of the CQT by giving a mix of students and law enforcement trainees the opprtunity to steal some money. Participants were allowed to choose for themselves whether or not to steal, making the simulation more realistic. They were then polygraphed under various different conditions, in which the researchers tested whether explaining the CQ in detail made a difference to the ability to discriminate guilty and innocent subjects.

Read on for the results.

Photo credit: pauldwaite, Creative Commons License

In Offe and Offe’s study, better accuracy rates in distinguishing guilty from innocent participants were achieved when examiners explain the purpose of the CQs in detail:

[…] identification rates of about 90% can be achieved for guilty as well as for innocent participants, even if they decide on their own whether they participate as guilty or as innocent. In light of these results, the criticism cannot be sustained that it would be impossible to systematically achieve a differential significance of relevant and comparison questions and to measure it physiologically. [Offe and Offe, p. 299]

Participants were also asked for their subjective assessments of the stress they felt answering both RQs and CQs, and Offe and Offe found that, in line with the theoretical logic of the test (as outlined above),

[…] RQs are felt to be more stressing than CQ by guilty participants, whereas innocent participants find the CQ more stressing. [p297]

Reference :

Abstract: In a mock crime study of the comparison question test (CQT), 35 subjects decided to participate as guilty and 30 as innocent. Two conditions were varied: Explaining the comparison questions in the pretest interview and re-discussing comparison questions between charts. Higher identification rates (?90% for guilty and innocent participants) were achieved in groups with explanation of comparison questions than in groups without explanation. Re-discussing comparison questions had no effect on identification rates. Ratings of subjective stress due to relevant and comparison questions were also obtained and can be seen as indicators of the significance of the questions. The significance of comparison questions was hardly affected by the different testing conditions. When effects are detectable at all, they contradict theoretical expectations in their direction. Results are discussed in terms of the significance of comparison questions used in polygraph testing.

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