Polygraph examination is often discussed in terms of test accuracy, question formulation, physiological recording, scoring systems, and examiner competence. These are all essential. However, one equally important issue receives less practical attention: the prior probability — the examiner's estimate of how likely the target behaviour, allegation, disclosure, or risk issue is before the polygraph result is considered.
If you have read our earlier articles on Reverse-Bayes reasoning and aggregation of prior probabilities, you will already understand that the prior probability is not a footnote in Bayesian analysis — it is a load-bearing element. This article turns that theoretical understanding into a practical workflow for examiners and the professionals who instruct them.
“Choosing the correct prior does not mean assuming guilt or innocence. It means asking a disciplined question: before considering the polygraph charts, what is the most defensible estimate of the likelihood that the relevant behaviour occurred?”
Why Prior Probability Matters
Polygraph results are not interpreted in a vacuum. Every examination takes place within a context — a referral question, a body of pre-existing information, a population base rate, and a set of professional expectations. The prior probability captures that context in a structured, quantifiable form.
- Context shapes interpretation. The same physiological data may carry different weight depending on whether the examination arises from a specific allegation with corroborating evidence, or from a routine screening programme in a low-risk population.
- Event-specific examinations differ from screening examinations. A single-issue diagnostic examination addressing a known allegation operates in a different statistical context from a multiple-issue screening designed to monitor compliance or risk.
- A 50/50 starting point is not always defensible. Defaulting to a 50% prior may feel neutral, but in many contexts it significantly overstates or understates the realistic likelihood.
- Suspicion should not be treated as evidence. The fact that an allegation has been made, or that a referral has been initiated, does not itself establish a high prior probability.
- The prior should be transparent and documented. An undocumented prior is an indefensible prior.
Event-Specific Polygraph Examinations
Event-specific examinations concern a known allegation, incident, offence, complaint, breach, or disclosure issue. The referral question is typically focused: did this specific behaviour occur?
Examples include alleged sexual contact, disputed workplace incidents, suspected licence breaches, prohibited material, theft, or contested relationship disclosures.
The examiner should ask: “What independent information exists before the polygraph is conducted?”
The prior in an event-specific examination should be informed by independent case information — not by the allegation itself. Key considerations:
- Strong, specific independent evidence (e.g. contemporaneous messages, digital records, witness accounts, admissions) may justify an elevated prior.
- Weak, ambiguous, or contested evidence should lead to a cautious or moderate prior range.
- No reliable independent evidence may require a neutral or broad prior range, with this uncertainty clearly documented.
The examiner must distinguish between the strength of the allegation and the strength of the independent evidence. An allegation may be serious and specific, but if no corroborating evidence exists, the prior should reflect that absence.
Screening, Monitoring and Risk-Management Examinations
Screening examinations are usually not based on one known incident. They are used to identify possible undisclosed behaviour, non-compliance, or risk. Examples include pre-employment screening, PCSOT monitoring, therapeutic disclosure work, sexual history disclosure, maintenance examinations, and high-risk occupational screening.
In screening contexts, a 50/50 prior is often inappropriate. The key question is: “What is the realistic base rate of this behaviour in this population?”
- Rare behaviours usually require a low prior. If the base rate of a target behaviour is 5% in the relevant population, starting from 50% dramatically overstates the pre-test likelihood.
- Higher-risk populations may justify an elevated prior, but only with clear reasoning and documented evidence.
- Uncertain base rates should be reported as uncertainty, not forced into a single number. A range (e.g. 10–25%) is more honest than a false point estimate.
Screening examinations contribute structured information to a wider process — they do not produce stand-alone verdicts. The prior should reflect this limited but valuable role.
Suspicion Is Not the Same as Evidence
This is one of the most important practical principles in prior selection.
“Suspicion can justify the referral. Evidence should justify the prior.”
Referral sources may have strong views about the examinee's truthfulness. That is often why they instruct the examination. However, the examiner's reasoning about the prior must remain independent of the referral source's beliefs.
Weak bases for setting the prior:
- “The partner believes he is lying.”
- “The employer is suspicious.”
- “The solicitor thinks the client is truthful.”
- “The referral source is convinced.”
Stronger bases for setting the prior:
- Materially inconsistent accounts across verified sources.
- Phone-location evidence or digital records.
- Contemporaneous messages or communications.
- Admissions or partial disclosures.
- Independent witness accounts.
- Previous relevant breaches or offending history.
- Documented treatment disclosures or probation records.
The examiner must separate allegation, suspicion, referral context, and independent evidence. Each may be relevant, but they carry different evidential weight.
The examiner's role is not to confirm the belief of the referral source. The examiner's role is to conduct a structured, ethical, evidence-informed assessment.
Multiple-Issue Screening Requires Special Care
Multiple-issue screening is more complex than single-issue testing. Each relevant issue may have a different prior probability, and it is poor practice to assume that all issues share the same prior.
Examples of possible multiple issues in a screening examination:
- Undisclosed sexual contact.
- Prohibited pornography use.
- Unauthorised victim contact.
- Breach of licence conditions.
- Alcohol or drug use.
- Unauthorised internet activity.
- Undisclosed offending behaviour.
Issue-level priors should be considered separately. Aggregation and multiplicity effects can affect interpretation: when several independent low-probability behaviours are examined together, the probability that at least one occurred may be substantially higher than any individual prior.
Issue-Level Checklist
For each relevant issue, the examiner should ask:
- Is this issue independent of the others?
- Is the behaviour rare, common, or unknown in this population?
- Is there case-specific evidence relevant to this issue?
- Is there a relevant population base rate?
- Is the prior for this issue different from the other issues?
- Would a result on one issue affect interpretation of another?
Using Prior Ranges Instead of False Precision
In many real-world cases, examiners do not have enough information to justify a precise numerical prior. A range is often more honest and more defensible than a single point estimate.
| Prior Range | Interpretation |
|---|---|
| Low prior | The behaviour is unlikely based on available evidence or base rate. |
| Low-to-moderate prior | There is some concern, but limited supporting evidence. |
| Moderate prior | The evidence or context is balanced or mixed. |
| Moderate-to-high prior | There is meaningful evidence or elevated contextual risk. |
| High prior | Strong independent evidence supports the likelihood of the behaviour. |
| Uncertain prior | Available information is insufficient, unreliable, or contested. |
Sample Reporting Language
“The prior probability was treated as uncertain because the referral information contained concern but limited independent corroboration. The examination result should therefore be interpreted cautiously and in conjunction with all other available information.”
“The prior probability was considered elevated due to the presence of independent case information, including inconsistent accounts and supporting digital evidence. The polygraph result is interpreted as one component of the wider evidential picture.”
“A neutral prior was considered because the available information did not provide a reliable basis for elevating or reducing the pre-test probability.”
A Practical Flow Chart for Choosing the Correct Prior
The following is offered as a practical decision aid for examiners, not as a rigid mathematical formula. Professional judgement remains essential at every stage.
Is there a known allegation, event, or identified incident?
Yes — Event-Specific
- Strong evidence → evidence-informed prior
- Weak evidence → cautious prior range
- No evidence → neutral or broad range
No — Screening / Monitoring
- Rare behaviour → low prior
- Higher-risk population → elevated prior
- Unknown base rate → range, not point
Are there multiple relevant issues?
If Yes
- Estimate the prior separately for each issue
- Consider aggregation and multiplicity effects
- Report issue-level and overall uncertainty
Final Steps
- Check for possible bias
- Document the source and rationale for the prior
- State assumptions and uncertainty explicitly
- Interpret the polygraph result cautiously and in context
This flow chart is a decision aid, not a substitute for professional competence. No formula or diagram can replace the examiner's responsibility to exercise informed, transparent, and ethically grounded judgement.
Bias Safeguards for Examiners
Referral information can influence examiner expectations, sometimes without conscious awareness. To maintain the integrity of the examination process:
- Distinguish allegation from evidence in all pre-test reasoning.
- Limit unnecessary case information where possible.
- Use structured pre-test procedures consistently.
- Keep question formulation standardised and behaviour-specific.
- Use validated scoring methods.
- Consider blind scoring or peer review in high-stakes cases.
- Document all assumptions, including the prior and its basis.
“The examiner's role is not to confirm the referral source's belief, but to conduct a structured, ethical, evidence-informed assessment.”
Practical Prior-Selection Checklist
- □ What is the exact referral question?
- □ Is this an event-specific, screening, monitoring, or therapeutic examination?
- □ Is there a known allegation or incident?
- □ What independent evidence exists?
- □ Is the evidence strong, weak, contested, or absent?
- □ What is the relevant population or context?
- □ Is the target behaviour rare, common, or unknown in this population?
- □ Am I using suspicion as evidence?
- □ Am I defaulting to 0.50 without justification?
- □ Are there multiple issues?
- □ Have I estimated the prior separately for each issue?
- □ Are the issues independent or connected?
- □ Would a range be more appropriate than a single prior?
- □ Could referral information bias the examination?
- □ Have I documented the source and uncertainty of the prior?
- □ Have I made clear that the polygraph result is one part of the wider assessment?
Conclusion
Prior probability is one of the most important but under-discussed aspects of polygraph interpretation. Choosing the correct prior helps prevent overconfidence, reduces bias, and improves professional decision-making.
“A polygraph result should not be interpreted in isolation. It should be interpreted in light of the referral question, the evidence, the relevant base rate, the number of issues tested, and the uncertainty surrounding the case.”
For polygraph science to be taken seriously in forensic, legal, and professional contexts, the prior probability must be treated as a first-order question — not an afterthought.
This article is provided for professional information only. A polygraph examination is not a clinical diagnosis, legal verdict, or guarantee of truthfulness. Results should be interpreted cautiously and in context. Where there are safeguarding, legal, or mental health concerns, appropriate professional advice should be sought.
Dr Keith Ashcroft is a Chartered Psychologist, polygraph examiner, and member of the American Polygraph Association at the Centre for Forensic Neuroscience. To discuss whether a structured polygraph examination may be appropriate for your circumstances, contact Dr Ashcroft for a confidential consultation.