A comparison of field and laboratory polygraphs in the detection of deception

This study compared the effectiveness of field versus laboratory polygraph instruments in discriminating guilty and innocent subjects using the Control Question Technique. Forty-eight prisoners were examined regarding a mock theft while physiological responses were simultaneously recorded on both a Lafayette field polygraph and a Sensor Medics laboratory polygraph, revealing essentially equivalent accuracy rates between the two systems.

Mock crime study with 48 prisoners tested using standard CQT procedures with simultaneous recording on both field (Lafayette) and laboratory (Sensor Medics) polygraph systems to directly compare instrumental performance.

Patrick and Iacono conducted simultaneous recordings of Control Question Tests using both field and laboratory polygraph instruments to determine if instrumental differences affected detection accuracy. The study used 48 prisoners in a mock theft scenario, with half guilty and half innocent participants. Results showed equivalent hit rates between field (79%) and laboratory (73%) polygraphs, with electrodermal activity proving the best discriminating measure for both systems. The findings suggested that instrumental sophistication was not the limiting factor in CQT accuracy, contradicting long-standing assumptions about the superiority of laboratory-grade equipment.

These findings validated the adequacy of field instruments used by practitioners and shifted focus from instrumentation limitations to the fundamental psychological processes underlying the Control Question Test. The results suggested that improving polygraph accuracy would require addressing test methodology rather than simply upgrading equipment.

Background & Context

The comparison between field and laboratory polygraph instruments represents a fundamental question in psychophysiological deception detection. Field polygraphs—portable analog instruments used by practitioners—differ substantially from research-grade laboratory systems in their sensitivity, recording capabilities, and measurement precision. This 1991 study addressed whether these instrumental differences affected detection accuracy when both systems simultaneously recorded Control Question Test (CQT) examinations.

Patrick and Iacono's research emerged during a critical period of scientific scrutiny regarding polygraph validity. The study built upon their previous work examining psychopathy and threat effects in polygraph testing, and represented one of the few attempts to directly compare field and laboratory instrumentation under standardized conditions. This comparison was particularly important because practitioners relied almost exclusively on field instruments, while skeptics argued that only laboratory-grade systems provided data suitable for scientific evaluation.

Research Design & Methodology

The study examined 48 prisoners who participated in a mock theft scenario within a peer threat context. Half the participants committed the mock theft (guilty group) while the other half did not (innocent group). This prison-based design provided higher ecological validity than typical laboratory studies using student volunteers.

The key methodological innovation involved simultaneous recording: physiological responses were captured concurrently on both a Lafayette field polygraph (the standard practitioner instrument) and a Sensor Medics laboratory polygraph. This allowed direct comparison of the same physiological events as recorded by both systems. All participants were tested using standard Control Question Technique procedures, ensuring consistency with real-world practice.

The study employed multiple scoring approaches: numerical scoring of field polygraph charts (the standard practice method) and quantitative measurement of physiological parameters from the laboratory system. This design enabled assessment of whether the superior measurement capabilities of laboratory instruments translated into improved discrimination between guilty and innocent participants.

Results & Key Findings

Overall hit rates were essentially the same whether classifications were based on quantitative measures from the laboratory polygraph (73%) or numerical scores from the field polygraph (79%). This finding contradicted the assumption that laboratory instruments would substantially outperform field polygraphs in deception detection.

The physiological measures proved equally important across both systems:

• Electrodermal activity emerged as the best discriminating measure for both polygraph types
• Indices of respiration suppression also showed strong discriminative power on both systems
• Cardiovascular measures contributed less consistently to accurate classification

A critical finding emerged regarding the nature of classification errors: most errors were attributable to the failure of available measures to differentiate between relevant and comparison questions, rather than to instrumentation limitations. This suggested that the fundamental challenge in polygraph testing lay not in recording fidelity but in the psychological processes underlying the CQT.

Discussion & Significance

These results challenged a long-standing debate in polygraph science. Critics had argued that field polygraphs lacked the precision necessary for valid psychophysiological assessment, while practitioners maintained that their instruments were adequate. Patrick and Iacono's findings suggested that instrumental sophistication was not the limiting factor in CQT accuracy—the 73-79% accuracy rates were comparable regardless of recording system quality.

The study's implications extended beyond instrumentation. The similar performance of both systems, combined with the finding that errors stemmed from poor question differentiation rather than measurement quality, pointed to more fundamental issues with the CQT methodology itself. The authors' work contributed to growing scientific consensus that the CQT's theoretical foundation—rather than its implementation—posed the primary validity concern.

This research became frequently cited in subsequent critiques of polygraph validity. The study has been referenced in discussions of field validity research and the limitations of confession-based ground truth verification, establishing it as an important contribution to the scientific literature on deception detection.

Limitations & Considerations

The prison-based mock crime design, while more realistic than typical laboratory studies, still differed substantially from actual criminal investigations. The "peer threat" context created motivation, but consequences remained limited compared to real criminal cases. Additionally, the immediate testing after the mock theft may not reflect the typical delay between crime and polygraph examination in field settings.

The overall accuracy rates of 73-79% leave considerable room for error, with innocent participants particularly vulnerable to misclassification. The study's focus on instrumental comparison meant that fundamental questions about CQT validity remained unaddressed. The finding that neither advanced laboratory instruments nor standard field polygraphs achieved highly accurate discrimination underscored concerns about the technique's reliability regardless of instrumentation quality.

Practical Applications

For polygraph practitioners, this research provided reassurance that field instruments were adequate for their intended purpose—the accuracy limitations of polygraph testing did not stem from inferior recording equipment. Examiners could not improve results substantially by investing in more sophisticated instrumentation. The emphasis should instead focus on test administration, question construction, and examiner training.

For consumers of polygraph services and legal decision-makers, the findings reinforced the importance of understanding that polygraph accuracy has inherent limits that better technology cannot overcome. The 73-79% accuracy range, consistent across both instrument types, suggests significant error rates that should inform how much weight polygraph results receive in investigative or legal contexts. Courts and agencies considering polygraph evidence should recognize that instrumental sophistication does not address the fundamental methodological challenges of the CQT.