Effect of two types of control questions and two question formats on the outcomes of polygraph examinations

Abstract:  Two major variations of polygraph “Control Question” testing, the Zone Comparison (ZoC) and the Modified General Question Test (MGQT) were evaluated. Within each, the type of control question, Exclusive or “time bar” (e.g., “Before you were 21, did you ever...”) and Nonexclusive or “no time bar”(e.g., “Did you ever….?”) was manipulated in a mock theft scenario, with 80 male and 40 female subjects randomly assigned to be either innocent or guilty. Polygraphic data collected by experienced field examiners were numerically scored by an evaluator blind to all aspects of the study. Decision accuracy was not related to the type of procedure (ZoC/MGQT) used or the subject’s sex. Accuracy was significantly related to the type of control question [χ2(2) = 11.46, p = 0.003; τc = 0.29]. Nonexclusive control questions produced greater accuracy than Exclusive control questions on both innocent and guilty subjects. These results and subjects’ self‐reports support the general “theory” on which control question (CQ) testing is based. The need for better empirical support of accepted dogma and current field practices is strongly indicated by these findings.

Randomized experimental design with 120 subjects (80 male, 40 female) assigned to guilty or innocent conditions in a mock theft scenario. Polygraph data collected by experienced field examiners were numerically scored by a blind evaluator. The study employed a 2x2 factorial design manipulating control question type and test format.

Horvath and Palmatier conducted a controlled laboratory study to empirically test whether field practices regarding control question formulation actually produce superior polygraph outcomes. Using a 2×2 factorial design, they manipulated control question type (exclusive vs. nonexclusive) and test format (ZoC vs. MGQT) across 120 participants in a mock theft scenario with known ground truth. Results demonstrated that nonexclusive control questions significantly outperformed exclusive questions (χ² = 11.46, p = 0.003), achieving 85% accuracy for guilty and 91% for innocent subjects. Test format and participant sex did not significantly influence accuracy, indicating that the fundamental control question structure matters more than procedural arrangement. These findings suggest that conventional wisdom favoring time-bar questions in some polygraph schools may not be empirically justified.

The study provides empirical validation that nonexclusive control questions should be preferred in CQT practice, contradicting some established field protocols and professional opinions. These results support the theoretical foundations of CQT and suggest that examiner training and standardization should prioritize control question formulation over procedural format variations.

Background & Context

The debate over optimal control question formulation represents one of polygraph's most persistent methodological controversies. Since the development of Control Question Test (CQT) procedures, examiners have debated whether comparison questions should include temporal restrictions ("exclusive" or "time bar" questions like "Before you were 21, did you ever...") or remain temporally unrestricted ("nonexclusive" questions like "Did you ever...?"). Despite decades of field practice and strongly held professional opinions, empirical research comparing these formats remained surprisingly limited.

This 2008 study by Horvath and Palmatier addressed a critical gap in polygraph science: testing whether accepted field practices actually produce superior outcomes. The research was supported in part by the School of Criminal Justice at Michigan State University and a grant from the American Polygraph Association to the APA's Credibility Assessment Research Center, reflecting the profession's recognition that empirical validation of standard techniques was overdue. The study also examined two widely-used CQT formats—the Zone Comparison Technique (ZoC) and the Modified General Question Test (MGQT)—to determine whether question format interacted with control question type.

Research Design & Methodology

The study employed a mock theft scenario with 120 participants (80 male and 40 female subjects) who were randomly assigned to be either innocent or guilty. This randomized experimental design allowed researchers to establish ground truth—a critical advantage over field studies where actual guilt or innocence often remains uncertain.

The experimental design manipulated two key variables in a factorial arrangement:

• Control question type: Exclusive (time-bar) questions versus Nonexclusive (no time-bar) questions
• Test format: Zone Comparison (ZoC) versus Modified General Question Test (MGQT)
• Participant characteristics: Gender was included as a variable to assess potential sex differences

Polygraphic data were collected by experienced field examiners and numerically scored by an evaluator who was blind to all aspects of the study, including participant guilt/innocence status, control question type, and test format. This blinding procedure was essential to eliminate scorer bias and ensure objective evaluation of the physiological data.

Results & Key Findings

Nonexclusive control questions produced greater accuracy than Exclusive control questions on both innocent and guilty subjects. This represented the study's most significant finding, challenging conventional wisdom in some polygraph schools that advocated exclusively for time-bar questions.

Key statistical findings:

• Accuracy was significantly related to the type of control question (χ² = 11.46, p = 0.003; τc = 0.29)
• Nonexclusive control questions achieved accuracy rates of 85% for guilty subjects and 91% for innocent subjects
• Decision accuracy was not related to the type of procedure (ZoC/MGQT) used or the subject's sex

The absence of significant differences between ZoC and MGQT formats was noteworthy, suggesting that the fundamental comparison question structure matters more than the specific arrangement of questions within the test. Similarly, the lack of sex differences indicated that both male and female participants responded comparably to polygraph examinations under controlled conditions.

Discussion & Significance

These results and subjects' self-reports supported the general "theory" on which control question (CQ) testing is based. The superior performance of nonexclusive questions aligns with CQT theory's premise that control questions must engage innocent examinees' concerns more effectively than relevant questions. Temporally unrestricted questions may be psychologically broader and more threatening to innocent individuals, creating stronger physiological responses that facilitate discrimination from deceptive examinees.

The study's findings contradicted some established field practices that emphasized exclusive control questions. The need for better empirical support of accepted dogma and current field practices was strongly indicated by these findings. This represented a significant challenge to the profession: techniques passed down through training programs and advocated in professional literature did not necessarily represent optimal practice when subjected to empirical testing.

The equivalence of ZoC and MGQT formats suggests that examiners have flexibility in test structure without sacrificing accuracy. This finding supports a more pragmatic, evidence-based approach to technique selection rather than rigid adherence to specific schools or methodologies. The research contributes to a growing body of literature demonstrating that core psychological principles matter more than superficial procedural variations.

Limitations & Considerations

Individuals engaging in research studies (most commonly college students) are often notably different from those involved in criminal justice contexts. Mock crime scenarios lack the high-stakes consequences, emotional intensity, and real-world complexity of actual criminal investigations. Laboratory participants face no genuine legal jeopardy, may be more compliant, and differ demographically from typical forensic populations.

Additional methodological considerations include the use of a single blind scorer, potential differences in how field examiners might conduct pretest interviews in operational settings, and the limited range of crime scenarios examined. The study also focused solely on numerical scoring outcomes without examining chart-level features or examiner decision-making processes that occur in actual practice. Replication in field settings with actual criminal investigations would strengthen confidence in these findings' generalizability.

Practical Applications

For practicing polygraph examiners, this research provides clear guidance: nonexclusive control questions appear to offer superior accuracy across both innocent and guilty examinees. Examiners trained in techniques emphasizing exclusive, time-bar comparison questions should consider whether their current approach optimizes accuracy or simply reflects tradition. The flexibility to use either ZoC or MGQT formats without compromising validity allows examiners to select formats best suited to specific case circumstances or institutional requirements.

For consumers of polygraph services and legal decision-makers, these findings underscore an important principle: polygraph accuracy depends on evidence-based technique selection, not simply examiner experience or credentials. The study demonstrates that even widely accepted practices benefit from rigorous empirical testing. Organizations employing polygraph should encourage or require examiners to use validated procedures, including nonexclusive control question formats when appropriate, rather than defaulting to tradition-based approaches that may compromise accuracy.