What is forensic science?

Courts frequently face questions that cannot be answered by witnesses: what a substance is, whether two samples share a common origin, whether a document has been altered, what a set of records shows. Forensic science exists to answer such questions by methods that can be described, checked, and challenged.

The requirement that the method be checkable is not incidental. Evidence that a court cannot examine is evidence a court cannot properly weigh, which is why disclosure of underlying data and access to independent expert review are treated as part of the discipline rather than as concessions.

Several structural features determine how much weight a forensic finding can bear.

The question asked

A finding answers a specific question about material. Moving from that finding to a conclusion about a person involves further inference that is a matter for the court, not for the scientist.

Measurement and interpretation

Some disciplines produce measurements; others rest on a trained examiner’s comparative judgement. Both can be reliable, but interpretive disciplines depend heavily on examiner proficiency, documented criteria, and independent verification.

Uncertainty

A responsible finding states its limits: what the result does not exclude, and what assumptions it depends on. A conclusion expressed without qualification is usually overstated.

Quality systems

Validated methods, accreditation, proficiency testing, and documented procedures are what allow a result to be relied on. Where they are absent, the result is weaker regardless of the technique used.

Continuity of material

A result is only as good as the account of how the material reached the laboratory. Chain-of-custody failures can remove evidence from a case entirely.

A major review by the National Research Council in 2009 examined forensic practice in the United States and found substantial variation between disciplines in scientific foundation, standardisation, and quality assurance, concluding that interpretation of evidence varied between jurisdictions. Its findings concern the United States as of 2009 and do not describe forensic practice in other countries or the position today.

Standards and scientific-foundation work has continued since. In the United States, the National Institute of Standards and Technology conducts foundation reviews of forensic methods and administers a body that develops forensic science standards. In Europe, forensic institutes participate in a network engaged in quality assurance and best-practice work, according to that network’s own description of itself.

• Common belief: Forensic evidence is objective and therefore conclusive.

  In practice: Many disciplines involve a trained examiner’s comparative judgement rather than a numerical output. That does not make them worthless, but it does mean examiner reliability, documented criteria, and verification are central rather than administrative details.

• Common belief: A match identifies a person.

  In practice: A finding of shared characteristics is a statement about material, expressed with a degree of confidence and subject to stated assumptions. Moving from that to identification of a person is an inference for the court, and the strength of that inference depends on the case as a whole.

• Common belief: All forensic disciplines have the same scientific standing.

  In practice: Major reviews have found that disciplines differ substantially in how well their underlying assumptions have been tested. Treating "forensic evidence" as a single category with a single reliability is one of the most consequential errors a reader can make.

• Common belief: Results are available quickly.

  In practice: Examination is a queued laboratory process with validation, verification, and reporting stages. The compressed timelines shown in fiction are among the most persistent misconceptions about the field.

• Common belief: An absence of forensic evidence means nothing happened.

  In practice: Material may never have been present, may not have been recoverable, or may not have been examined. Absence of a finding is not a finding of absence, and courts are generally directed accordingly.

• Admissibility standards for expert evidence differ substantially between legal systems, including how far a court examines the reliability of the underlying method.
• Some systems rely on court-appointed experts; others on experts instructed by the parties. This changes how disagreement between experts is resolved.
• Accreditation requirements differ: mandatory for certain disciplines in some countries, voluntary in others.
• Forensic services may be delivered by state laboratories, by police-operated laboratories, by universities, or by commercial providers, and the arrangement affects both independence and quality assurance.
• Whether the defence can obtain independent examination of the same material, and who pays, varies considerably and materially affects the ability to challenge a finding.

Forensic evidence is meant to be contestable. The mechanisms that make it so are the accountability mechanisms of the discipline.

• Disclosure of underlying data, notes, and method, not only the conclusion.
• Access to independent expert review, which requires both the material and the funding to examine it.
• Admissibility rules allowing a court to exclude evidence whose method is insufficiently reliable.
• Accreditation and proficiency testing, which make examiner and laboratory performance measurable rather than assumed.
• Post-conviction review where a discipline’s foundation is later re-examined, which has led to convictions being reopened in several jurisdictions.

Related: what a criminal investigation is, what courts do, and why justice systems need oversight.