Unit 4: Types and Classification of Evidence

Forensic science treats evidence as any object, substance or record that helps establish whether a crime occurred and who was involved (the discipline crystallised as a system after Edmond Locard opened the first police laboratory in Lyon, 1910). Evidence is the material link between a suspect, a victim and a scene, and every classification below rests on a single principle.

• Locard's Exchange Principle: every contact leaves a trace — when two surfaces meet, material transfers both ways, so a perpetrator both deposits and carries away matter.
• Two evidential roles: corroborative evidence supports a witness or theory; associative evidence physically links a person to a scene or item.
• Chain of custody: the documented, unbroken record of who handled an item, when and where, without which evidence is inadmissible.
• Probative value: the weight evidence carries in proving a fact, which rises sharply when a trait is rare rather than common.
• Comparison logic: most conclusions come from matching a questioned sample (from the scene) against a known or reference sample (from a suspect or source).

II. Physical Evidence — Tangible Material at the Scene

A. Definition

Physical evidence is any tangible object with mass and dimension that can be collected, preserved and examined to reconstruct events or establish associations.

• Scope: ranges from a firearm or shoe to a paint chip or a bloodstain — anything perceptible and measurable.
• Contrast with testimonial evidence: unlike a witness statement, physical evidence is mute but incorruptible; it does not lie, forget or change under pressure.

B. Nature

Its defining nature is objectivity and persistence, but it demands careful handling to retain integrity.

• Objective: properties (mass, refractive index, blood type) exist independently of any observer's opinion.
• Transferable and fragile: obeys Locard's principle, so evidence can be added, lost or contaminated between deposition and collection.
• Latent or patent: may be visible (a bloody knife) or hidden until enhanced (a fingerprint developed with ninhydrin).
• Reconstructive: the position and condition of items — e.g. the spread of a bloodstain pattern — reveals sequence and mechanism of events.

C. Significance

Physical evidence matters because it can prove a corpus delicti, identify or exclude a suspect, and corroborate or refute statements.

• Individualisation vs. identification: identification names what a substance is (this is heroin); individualisation ties it to one unique source (this bullet was fired from this barrel).
• Exculpatory power: can exonerate — DNA has overturned wrongful convictions where a scene sample excluded the accused.
• Reconstruction: trajectory rods, glass fracture sequence and blood spatter angle establish how and in what order events unfolded.

III. Trace Evidence — Minute Transferred Material

A. Definition

Trace evidence is physical evidence present in small or microscopic quantities, transferred between people, objects or environments during contact.

• Typical examples: hairs, textile fibres, glass fragments, soil, paint, gunshot residue and pollen.
• Micro-scale: often invisible to the naked eye and recovered by tape-lifting, vacuuming or forceps under magnification.

B. Nature

Its nature is direct dependence on Locard exchange, making it powerful but easily lost.

• Cross-transfer: a fibre moves from a car seat to clothing and a hair from the victim to the assailant in the same struggle.
• Class-bound: most trace material carries only class characteristics — a fibre is identified as nylon of a given dye and cross-section, not as one unique thread.
• Highly perishable: low mass means it can shed, blow away or contaminate; hence rapid, sealed collection.

C. Significance

Trace evidence links people and places even when no large object is left behind.

• Association: matching glass fragments on a suspect to a broken window (comparing refractive index and elemental profile via SEM-EDS) places the suspect at the scene.
• Investigative lead: pollen or soil mineralogy can indicate a geographic origin.
• Corroboration: paint transfer in a hit-and-run confirms contact between two vehicles.

IV. Pattern Evidence — Impressions and Reproducible Marks

A. Definition

Pattern evidence is evidence produced by physical contact between two surfaces that leaves a reproducible impression, mark or distribution.

• Categories: fingerprints, shoe and tyre impressions, toolmarks, firearm striations, bite marks, and bloodstain patterns.
• Two-dimensional or three-dimensional: a fingerprint in ink is 2-D; a shoe impression in soft mud is 3-D.

B. Nature

Its nature lies in the transfer of surface features from a source to a receiving medium, which can then be compared side by side.

• Reproducibility: the same shoe repeatedly prints the same wear pattern, allowing comparison of questioned and test impressions.
• Positive, negative and impressed: a print may add material (a bloody shoeprint), remove material (dust print), or deform a plastic surface (a bite mark).
• Reconstructive geometry: bloodstain shape encodes impact angle.

sin θ = W / L
θ = angle of impact of a blood droplet
W = width of the elliptical stain
L = length of the elliptical stain

C. Significance

Pattern evidence supports both individualisation and event reconstruction.

• Individualisation: ridge minutiae (bifurcations, ridge endings) let a latent fingerprint be attributed to one person.
• Sequence reconstruction: overlapping shoeprints and directional spatter reveal movement and order of events.
• Class-level screening: a tyre tread pattern narrows the suspect vehicle to a make and model before individualising wear features are examined.

V. Classification of Evidence — Grouping by Origin and Composition

Evidence is sorted by the type of information it carries and how it is analysed. The four principal classes below overlap — a single bloodstain is simultaneously physical and biological.

A. Biological evidence

Evidence originating from living organisms and analysed for its genetic or serological content.

• Examples: blood, semen, saliva, hair with root, bone, tissue and botanical material.
• Analysis: DNA profiling by STR (short tandem repeat) typing yields a profile whose random-match probability can reach one in billions.
• Significance: offers the highest individualising power of any evidence class.

B. Physical evidence

Evidence characterised by measurable physical properties rather than chemical or genetic makeup.

• Examples: glass, soil, firearms, tool marks, impressions and documents.
• Analysis: density, refractive index, hardness and morphology under microscopy.
• Significance: supports both class association and, through pattern matching, individualisation.

C. Chemical evidence

Evidence identified and compared by its chemical composition.

• Examples: drugs, explosives, accelerants, paint, ink and gunshot residue.
• Analysis: gas chromatography–mass spectrometry (GC-MS) separates and identifies compounds by retention time and mass spectrum; FTIR identifies functional groups.
• Significance: confirms identity of controlled substances and links residues to a source batch.

D. Digital evidence

Data stored or transmitted in binary form that is relevant to an investigation.

• Examples: files, emails, call logs, GPS records, browser history and CCTV footage.
• Analysis: forensic imaging (a bit-for-bit copy) preserved with a hash value (e.g. SHA-256) so any alteration is detectable.
• Significance: establishes timelines, locations and communications; its volatility makes write-blocking and hashing essential.

VI. Individual and Class Characteristics of Evidence

A. Framing distinction

Every trait an examiner observes is graded by how tightly it can be tied to a single source, which determines whether evidence merely narrows a group or names one origin.

• Comparison basis: an examiner compares questioned and known samples for both shared class traits and shared individual traits before drawing a conclusion.

B. Individual characteristics

Traits that can be attributed to a single, unique source to the exclusion of all others.

• Definition: features arising from random events — manufacturing flaws, wear, injury — that are effectively unique.
• Examples:
  • Fingerprints: the specific arrangement of ridge minutiae, unique even between identical twins.
  • Firearm striations: imperfections in a barrel scratch a signature onto every bullet fired.
  • DNA STR profile: the combination of allele repeat numbers across loci.
• Result: permits individualisation — a positive source attribution.

C. Class characteristics

Traits common to a group of items sharing a common origin, allowing a source group but not one item.

• Definition: properties set by design or manufacture, shared by all members of a category.
• Examples:
  • Blood type: ABO group A is shared by a large population fraction.
  • Fibre type: polyester of a given denier and dye is common to a production run.
  • Shoe size and tread design: shared by every pair of that model.
• Result: narrows the field and can exclude a source, but cannot alone individualise.

D. Interplay in casework

The two categories work together, individual traits confirming what class traits first suggest.

• Cumulative strength: several independent class matches (fibre type and dye and blood group) multiply to strong associative value even without individual traits.
• Escalation: a shoe impression first classed by tread pattern becomes individualising once a nick or wear feature unique to one shoe is matched.
• Exclusion priority: a single mismatched class trait defeats an association outright, so class comparison is the efficient first filter.