UNIT 5: PHYSICAL EVIDENCE
PHYSICAL EVIDENCE
• INCLUDES ANY AND ALL OBJECTS THAT CAN ESTABLISH THAT A
CRIME HAS OR HAS NOT BEEN COMMITTED OR CAN LINK A CRIME
AND ITS VICTIM OR IT PERPETRATOR.
• USED TO HELP INVESTIGATORS RECONSTRUCT THE CRIME SCENE
• THE PURPOSE OF RECOGNIZING PHYSICAL EVIDENCE IS SO THAT IT
CAN BE COLLECTED AND ANALYZED.
• ALMOST ANYTHING COULD BE CONSIDERED PHYSICAL
EVIDENCE BUT THERE ARE A FEW COMMON CATEGORIES:
COMMON CATEGORIES OF PHYSICAL EVIDENCE
• BIOLOGICAL
• BLOOD, SEMEN, SALIVA, BONES, OTHER BODY FLUIDS
• IMPRESSION
• FINGERPRINTS, TOOL MARKS, TREAD PATTERNS, SHOE PRINTS
• MANUFACTURED
• POWERED RESIDUE, DRUGS, EXPLOSIVES, PLASTICS,
PETROLEUM PRODUCTS, PAINT, GLASS, FIBERS,
• DOCUMENTS, SERIAL NUMBERS
COMMON TYPES OF PHYSICAL EVIDENCE
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BLOOD, SEMEN, AND SALIVA
DOCUMENTS
DRUGS
EXPLOSIVES
FIBERS
FINGERPRINTS
FIREARMS AND AMMUNITION
GLASS
HAIR
IMPRESSIONS
• ORGANS AND PHYSIOLOGICAL
FLUIDS
• PAINT
• PETROLEUM PRODUCTS
• PLASTIC BAGS
• PLASTIC, RUBBER, AND OTHER
POLYMERS
• POWDER RESIDUES
• SOIL AND MINERALS
• TOOL MARKS
• VEHICLE LIGHTS
• WOOD AND OTHER VEGETATIVE
MATTER
EXAMINATION OF PHYSICAL EVIDENCE
• PHYSICAL EVIDENCE IS EXAMINED BY A FORENSIC SCIENTIST FOR
IDENTIFICATION AND COMPARISON
• IDENTIFICATION HAS, AS ITS PURPOSE, THE DETERMINATION OF THE
PHYSICAL OR CHEMICAL IDENTITY OF A SUBSTANCE WITH AS NEAR
ABSOLUTE CERTAINTY AS EXISTING ANALYTICAL TECHNIQUES WILL
PERMIT.
• FOR EXAMPLE: DID A BLOODSTAIN FOUND AT
THE SCENE COME FROM A HUMAN RATHER THAN
A DOG OR CAT?
• A COMPARISON ANALYSIS SUBJECTS A SUSPECT SPECIMEN AND A
STANDARD/REFERENCE SPECIMEN TO THE SAME TESTS AND
EXAMINATIONS FOR THE ULTIMATE PURPOSE OF DETERMINING
WHETHER OR NOT THEY HAVE A COMMON ORIGIN.
IDENTIFICATION
• REQUIRES THE ANALYSIS OF SPECIFIC PHYSICAL OR
CHEMICAL SUBSTANCES TO THE EXCLUSION OF ALL
OTHER POSSIBLE SUBSTANCES.
• THE PROCESS OF IDENTIFICATION FIRST REQUIRES THE
ADOPTION OF TESTING PROCEDURES THAT GIVE
CHARACTERISTIC RESULTS FOR SPECIFIC STANDARD MATERIALS.
• ONCE THESE TEST RESULTS HAVE BEEN ESTABLISHED, THEY MAY
BE PERMANENTLY RECORDED AND USED REPEATEDLY TO PROVE
THE IDENTITY OF SUSPECT MATERIALS.
• FOR EXAMPLE: WHEN TESTING UNKNOWN POWDER TO DETERMINE
IF IT IS HEROIN, TEST RESULTS MUST BE IDENTICAL TO THOSE OF A
KNOWN HEROIN SAMPLE
IDENTIFICATION, CONT’D
• SECOND, IDENTIFICATION REQUIRES THAT THE
NUMBER AND TYPE OF TESTS NEEDED TO
IDENTIFY A SUBSTANCE BE SUFFICIENT TO
EXCLUDE ALL OTHER SUBSTANCES.
• EACH TYPE OF EVIDENCE REQUIRES DIFFERENT TESTS FOR
IDENTIFICATION AND TESTING METHODS WILL HAVE VARYING
DEGREES OF SPECIFICITY
• ONE SUBSTANCE MAY BE IDENTIFIED ACCURATELY WITH ONE TEST WHILE
OTHERS MAY REQUIRE A COMBINATION OF 5 OR 6 TESTS
• ETHICS, REMEMBER, MANDATE NO UNNECESSARY TESTING JUST TO
ACHIEVE A DESIRED RESULT
COMPARISON
• A COMPARATIVE ANALYSIS HAS THE IMPORTANT
ROLE OF DETERMINING WHETHER OR NOT A
SUSPECT SPECIMEN AND A STANDARD/REFERENCE
SPECIMEN HAVE A COMMON ORIGIN.
• BOTH THE STANDARD/REFERENCE AND THE SUSPECT
SPECIMEN ARE SUBJECT TO THE SAME TESTS.
• FOR EXAMPLE: FORENSIC SCIENTISTS MAY LINK A SUSPECT TO A
CRIME BECAUSE HAIRS FOUND AT THE SCENE (EVIDENCE) ARE SIMILAR
TO HAIRS REMOVED FROM THE SUSPECT’S HEAD (KNOWN
STANDARD)
COMPARISON, CONT’D
• THE FORENSIC COMPARISON IS ACTUALLY A
TWO-STEP PROCEDURE.
• FIRST, COMBINATIONS OF SELECT PROPERTIES ARE CHOSEN FROM
THE SUSPECT AND THE STANDARD/REFERENCE SPECIMEN FOR
COMPARISON.
• SECOND, ONCE THE EXAMINATION HAS BEEN COMPLETED, THE
FORENSIC SCIENTIST MUST BE PREPARED TO RENDER A CONCLUSION
WITH RESPECT TO THE ORIGINS.
• COMPARED FOR CLASS AND INDIVIDUAL CHARACTERISTICS:
INDIVIDUAL CHARACTERISTICS
• EVIDENCE THAT CAN BE ASSOCIATED WITH A COMMON SOURCE
WITH AN EXTREMELY HIGH DEGREE OF PROBABILITY
• EVIDENCE WITH INDIVIDUAL CHARACTERISTICS:
 FINGERPRINTS, DNA, ETC.
 IRREGULAR AND RANDOM WEAR PATTERNS IN TIRE OR FOOTWEAR IMPRESSIONS
 STRIATIONS (RIFLING) ON BULLETS OR TOOL MARKS
 FITTING THE IRREGULAR EDGES OF BROKEN OBJECTS TOGETHER (JIGSAW)
 OR, MATCHING SEQUENTIALLY MADE PLASTIC BAGS BY STRIATION MARKS ALONG THE BAGS
• FOR EXAMPLE: MATHEMATICAL CALCULATIONS HAVE SHOWN THE
PROBABILITY OF 2 INDIVIDUALS HAVING THE SAME FINGERPRINTS TO BE
ONE IN 1 X 1060
CLASS CHARACTERISTICS
• EVIDENCE ASSOCIATED ONLY WITH A GROUP IS SAID TO HAVE CLASS
CHARACTERISTICS.
• SOMETIMES FORENSIC INVESTIGATORS CANNOT RELATE PHYSICAL EVIDENCE TO
A SINGLE SOURCE WITH A HIGH DEGREE OF CERTAINTY…BUT RATHER A GROUP
OF INDIVIDUALS….CLASS CHARACTERISTICS
• EXAMPLES OF CLASS EVIDENCE
 A, B, O BLOOD TYPES
• PROBABILITY IS ALSO A DETERMINING FACTOR
 IF WE COMPARE 2 SINGLE-LAYER AUTOMOBILE PAINT CHIPS …COMMON …COULD
BE NARROWED DOWN TO A GROUP
VERSUS 2 SEVEN-LAYER AUTOMOBILE PAINT CHIPS… LESS COMMON…
MORE EASILY NARROWED DOWN TO A SMALLER GROUP … HAS A HIGHER
PROBABILITY OF BELONGING TO THE SAME CAR
CALCULATING PROBABILITY
• EXAMPLE IS FROM THE TEXTBOOK PG. 91
• WHICH CASE MOST STRONGLY SUGGEST THE SUSPECT IS GUILTY ?
EX : .32 X .43 X .05 = .00688 X 100 = .68
THE ANSWER IS CASE 3 ….
.50
ASSESSING THE VALUE OF PHYSICAL EVIDENCE
• IN ALL CASES, IT IS NOT POSSIBLE TO STATE WITH
MATHEMATICAL EXACTNESS THE PROBABILITY THAT THE
SPECIMENS ARE OF COMMON ORIGIN.
• IT CAN ONLY BE CONCLUDED THAT THIS PROBABILITY IS
SO HIGH AS TO DEFY MATHEMATICAL CALCULATIONS OR
HUMAN COMPREHENSION.
LIMITATIONS OF PHYSICAL EVIDENCE
• ONE OF THE CURRENT WEAKNESSES OF FORENSIC SCIENCE IS THE INABILITY OF THE
EXAMINER TO ASSIGN EXACT OR EVEN APPROXIMATE PROBABILITY VALUES TO THE
COMPARISON OF MOST CLASS PHYSICAL EVIDENCE.
• FOR EXAMPLE, WHAT IS THE PROBABILITY THAT A NYLON FIBER ORIGINATED FROM A PARTICULAR
SWEATER, OR THAT A PAINT CHIP CAME FROM A SUSPECT CAR IN A HIT AND RUN?
• MOST ITEMS OF PHYSICAL EVIDENCE RETRIEVED AT CRIME SCENES CANNOT BE LINKED
DEFINITIVELY TO A SINGLE PERSON OR OBJECT.
• THE VALUE OF CLASS PHYSICAL EVIDENCE LIES IN ITS ABILITY TO PROVIDE
CORROBORATION OF EVENTS WITH DATA THAT ARE, AS NEARLY AS POSSIBLE, FREE OF
HUMAN ERROR AND BIAS.
LIMITATIONS, CONT’D
• THE CHANCES ARE LOW OF ENCOUNTERING TWO
INDISTINGUISHABLE ITEMS OF PHYSICAL EVIDENCE AT A CRIME
SCENE THAT ACTUALLY ORIGINATED FROM DIFFERENT SOURCES.
• WHEN ONE IS DEALING WITH MORE THAN ONE TYPE OF CLASS
EVIDENCE, THEIR COLLECTIVE PRESENCE MAY LEAD TO AN
EXTREMELY HIGH CERTAINTY THAT THEY ORIGINATED FROM THE
SAME SOURCE.
• FINALLY, THE CONTRIBUTION OF PHYSICAL EVIDENCE IS ULTIMATELY
DETERMINED IN THE COURTROOM.