Physical evidence Trace evidence

Small or microscopic materials that can be transferred upon contact – Hair, fiber, paint, glass, plastic, soil and pollen Examine known and questioned samples – Match (same source of origin) – Consistent (similar, cannot exclude) – Exclude (dissimilar in one or more ways) Class and individual characteristics – Class: Physical details common to all objects of a certain type – Individual: Details unique to a particular object, due to wear, contact with foreign materials, etc.

Comparison techniques Compare first on class characteristics, then on individual characteristics Gross appearance – Configuration – Size, weight, color Composition – Physical (e.g., strands, fibers) – Chemical Fractured / cut-end / segment analysis – Did fragments originate from a common source? – Were fragments once a single piece? The greater the similarities, the better the match – Be conservative! Any positive dissimilarity excludes

Hair Class characteristic or individual (with DNA) Origin: human or animal Gross appearance: color, length, shape, area of body, race of donor Composition: soap, dye and drug residues, cigarette smoke Manner of removal DNA analysis only way to exact match (requires root)

Fibers (incl. string, cord, rope, tape) Normally class-characteristic Gross appearance – Diameter, color, shade Composition – cotton, wool, blends, etc. – Construction (no. of strands, type of weave, etc.) Broken end comparison can identify a questioned fragment as having come from a known strand

Fibers - examples Strangulation: strands from rope imbedded in victim’s skin and clothes Burglary: carpet fibers on suspect’s clothes and shoes Bombing: cut end from roll of tape in suspect’s garage matched to tape end on pipe bombs

Paint and coatings Normally class-characteristic Vehicles – can identify color, make, model and year Gross appearance: color, weathering, layers Composition: chemistry Questioned chips can be matched to known source through broken end comparisons and/or layer analysis

Paint and coatings - examples Hit and run – Transfer from known vehicle to suspect vehicle – Transfer from known vehicle to victim’s clothing Burglary – Transfer from safe to pry bar Murder, body relocated – Drywall smear to victim’s clothing during violent struggle

Glass Fractured end comparison can be used to match questioned fragments to a known source Normally class-characteristic Gross appearance – Thickness, pattern Composition – Density – Refractive index – Light dispersion

Glass - examples Hit and run – Fragments from headlamps of questioned vehicle at scene – Broken end comparison possible Burglary – Glass fragments in suspect’s clothing

Soil Decayed rock, minerals, decomposed plant matter Normally class-characteristic Gross appearance – Color – Texture Composition – Chemicals – Pollen, contaminants Nearby samples can differ Alibi – collect from areas suspect normally frequents

Soil - examples Murder – soil from gravesite on suspect’s shoes – soil from victim’s yard tracked into suspect’s apartment Burglary – soil from area outside window on suspect’s shoes – soil from suspect’s yard tracked on victim’s carpet