What “confirmation bias” means in a DWI investigation

Confirmation bias is the human tendency to give greater weight to information that supports an existing expectation and less weight to information that points another way. The concept does not mean that an officer acted dishonestly or deliberately ignored evidence. It is a reason to examine the full sequence of an investigation instead of treating any one conclusion in a police report as self-proving.

By the time standardized field sobriety tests begin, an officer may already have observed driving, heard an explanation for the stop, smelled alcohol, noticed speech or eye appearance, received an admission, or used a roadside alcohol screening device. Those earlier observations may reasonably inform the investigation. They also create an expectation that can influence which later details are noticed, remembered, emphasized, or described.

The standardized battery and the importance of standardization

NHTSA’s current SFST curriculum identifies a three-test standardized battery: horizontal gaze nystagmus, the walk-and-turn, and the one-leg stand. The walk-and-turn and one-leg stand are divided-attention tests: the subject must remember instructions while performing physical tasks. The training materials emphasize prescribed instructions, demonstrations, positioning, timing, observation, and clue scoring.

“Standardized” does not mean every roadside setting is identical, and it does not guarantee that every clue proves impairment. It means that the administration and interpretation should be compared with the trained method. A deviation may be important—or it may be minor—depending on the facts and how it affects the reliability or weight of the observation.

How expectation can shape observation

A DWI investigation is cumulative. North Carolina’s impaired-driving statute, G.S. 20-138.1, permits the State to proceed under different theories, including driving while under the influence of an impairing substance or with a qualifying alcohol concentration. Field observations may be offered with driving evidence, statements, chemical testing, and other circumstances.

Potential confirmation-bias issues often appear in the transition from raw observation to report language. For example, a video might show a person briefly moving a foot, while the report summarizes the entire test as poor balance. Or the report may record details consistent with impairment but omit equally visible details such as clear speech, correct responses, a medical explanation, or performance that complied with most instructions. None of those facts automatically decides the case; the point is to preserve and compare the entire record.

What a careful DWI evidence review should compare

• Dash-camera and body-camera video from before, during, and after testing
• The officer’s exact instructions and demonstrations
• The location, grade, surface, lighting, weather, and passing traffic
• Footwear, age, balance limitations, injuries, medications, fatigue, and anxiety
• Which standardized clues were recorded and how each was counted
• Differences among video, handwritten notes, reports, and testimony
• Statements made by the driver and the context in which they were made
• Roadside screening, breath, blood, or urine evidence and its timing

Video is especially useful because it lets counsel test the words in a report against an independent record. But video also has limits: it may not capture the eyes well enough for an HGN review, its perspective can distort distance, sound may be incomplete, or part of the encounter may occur off camera.

Practical steps after a North Carolina DWI charge

1. Preserve documents and deadlines. Keep the citation, charging papers, civil-revocation paperwork, bond conditions, and every notice from the court or DMV.
2. Write down the sequence while it is fresh. Record road and weather conditions, instructions, footwear, medical issues, witnesses, medications, food, sleep, and the timing of events.
3. Do not assume the report is the full evidence file. Video, dispatch information, photographs, chemical-test records, and training or maintenance records may require separate requests and legal analysis.
4. Get case-specific advice promptly. Court, DMV, privilege, assessment, and evidence-preservation timelines can overlap.

For a broader explanation of the testing framework, read Standardized Field Sobriety Testing in North Carolina and explore the firm’s North Carolina Traffic & DWI Resource Center.