Criminalistics and Forensic Science
Forensic disciplines
Criminalistics applies science to legal questions. Core areas include fingerprints, questioned documents, ballistics, serology and DNA, toxicology, trace evidence, forensic photography, digital forensics, and pathology. Locard's exchange principle holds that contact leaves traces. Evidence has class characteristics shared by a group and individual characteristics unique to a source. The laboratory can associate or exclude; it rarely reconstructs the entire event. Chain of custody, proper collection, controls, and validated methods determine evidentiary value. Expert testimony must be relevant, reliable, and within the expert's field.
Traps
Do not overstate results. A fingerprint match identifies a source, not when or how a print was left. DNA can be deposited indirectly and can persist; a match is not automatic proof of guilt. Ballistics may associate a bullet with a firearm, not necessarily identify the shooter. Field tests are presumptive; confirmatory tests are needed. Gunshot residue can indicate discharge, not who discharged. Questioned-document examination may detect alterations but cannot always identify the writer. Avoid contamination and broken seals.
Worked example
A positive presumptive test for blood at a scene is not conclusive. The analyst should perform a confirmatory test and, if relevant, DNA typing with appropriate controls. Rule of thumb: presumptive screening first, confirmatory analysis second, interpretation last. State the limits of the finding in the report.