Presentation on theme: "Chapter 3 – Physical Evidence"— Presentation transcript:
1 Chapter 3 – Physical Evidence List the common types of physical evidence encountered at crime scenesExplain the difference between the identification and comparison of physical evidenceDefine individual and class characteristicsDiscuss the value of class evidence to a criminal investigationExplain the purpose physical evidence plays in reconstructing the events surrounding the commission of the crime scene.
2 Common Types of Physical Evidence Blood, semen, and saliva - identity and possible originLiquid or driedHuman or AnimalDocuments – authenticity or sourceHandwritingTypewritingDrugs –Any seized substance
3 Common Types of Physical Evidence ExplosivesDevices containing explosive chargeObjects suspected to contain residues of explosiveFibers – transfers establish relationshipsNaturalSyntheticFingerprintsLatentVisible
4 Common Types of Physical Evidence Firearms & AmmunitionFirearmDischarged or intact ammunitionGlassGlass particle or fragmentWindowpanes w/ holes made by bulletHairAnimalhuman
5 Common Types of Physical Evidence ImpressionsTire markingsShoe printsDepressions in soft soilsGlove, fabric impressionsBitemarks in skin, foodOrgans and physiological fluidTest existence of drugs or poisonsBlood analyzed for alcohol or drugsPaintLiquid or driedTransferred from one surface to another
6 Common Types of Physical Evidence Petroleum productsGasoline – residues from arsonGrease and oil stainsPlastic BagsPolyethelene disposable bagsPlastic, rubber, other polymersRemnants of man-made materialsPowder ResiduesItem suspected of containing firearm discharge residues
7 Common Types of Physical Evidence Serial numbersStolen property for restoration of erased I.D. #Soil and MineralsImbedded in shoesSafe insulation in garmentsTool MarksScrewdriver or crowbar impressed into another objectVehicle LightsLight on or off at time of impact
8 Common Types of Physical Evidence Wood & other vegetative matterWoodSawdustShavingsVegetative matterOn shoes, clothing, or tools
9 IdentificationUltimate Goal in identifying a specific physical or chemical substance is exclude other substancesRequires adoption of testing procedures that have characteristic results for specific standard materialsOnce established, procedure can be used repeatedly to prove identityRequires that # and type of tests needed to identify a substance be:Sufficient to exclude all other substancesI.E. eliminate all but one substance
10 IdentificationDetermination of physical or chemical identity of a substance with as near absolute certainty as possibleIllicit drug preperation – heroin, cocaine, barbituratesGasoline residue from debris of a fireNature of explosive residues – dynamite or TNTSpecies of Origin – blood, semen, hair, wood
11 IdentificationEach type of evidence requires a different test; each test has a different degree of specificitySubstance A – one testSubstance B – 5 or 6 testsNo control over quality or quantity of specimensForensic scientist determines at what point analysis is concludedHas to be beyond reasonable doubt for court of law
12 ComparisonSuspect specimen and a standard/reference specimen run through same tests, examinations to determine common originHairs at crime scene to hairs in suspect headPaint chip from victim’s clothing to car in hit-and-run
13 Comparison Procedure: Combinations of select properties chosen from suspect and standard/reference sampleWhich properties and how many properties depends on type of material (more details later….)Once testing complete, forensic scientist concludes on origins of specimenOne or more properties doesn’t agree – not same originAll properties agree – is it always the same origin?
14 Comparison Not necessarily always the same Evidential value – probability in ascertaining origins of two or more specimensProbabilityFrequency of occurrence of an eventOdds at which a certain event will occurWhat does this mean?Variation of characteristics of each specimen need to be taken into accountIndividual vs. Class Characteristics
15 Class Characteristics The properties that all the members of a certain group of objects or substances have in commonFrequent problem: inability of lab to relate physical evidence to a common origin with a high degree of certaintyProbability is a determining factor
16 Class Characteristics Initial categories are broad and then narrowed down as more information is obtainedExampleBlue substanceBlue paintBlue car paintBlue car paint from Ford Manufacturing CompanyBlue car paint from Ford producedBlue car paint from Ford produced , used on Mustang and Explorer
17 Class Characteristics The previous “paint” example was a refined class evidence identificationThe “paint” was identified by comparing its class characteristics with those of known standards or previously established criteria.The “paint” is considered CLASS EVIDENCE
18 Other Examples… Single layered paint Soil Glass fragments too small to fit back togetherHairsFibers
19 Class Characteristics ProbabilityTwo paint chips with one layer of similar colorTwo paint chips with 7 similar paint layers, not all part of car’s original colorWhich has high probability of same origin?
20 Class Characteristics Blood – 2 samples, both human origin, both type AType A is 26% occurrence in population – not enough for comparison -BUT-Use more blood factors to compare – probability increasesBlood proteins, growth factors, clotting factors, enzymes
21 Class Characteristics Product Rule – calculates overall frequency of occurrence in a populationFactor A % occurrence * Factor B % occurrence= probability that both events will occur simultaneouslyApplies when using factors that occur independently of one another
22 Class Characteristics Product Rule Example:Blood Factors FrequencyA %EsD %PGM %Multipy (.26) (.85) (.02)= .44% or 1 in 200Probability that a blood sample will contain all three types - .44% or 1 in 200 people
23 Value of Class Evidence Many lawyers try to discredit class evidence because it cannot be limited to just one possible sourceHowever Class Evidence DOES HAVE VALUE.Look around room, most have different clothes on. If I were to pick a fiber from one persons clothes in this room, that fiber could possibly pick out the single source or at least narrow the fieldSome class evidence holds little forensic value such as fiber from jeans or white cotton shirts…they are too common
24 Individual Characteristics Properties of evidence that can be attributed to a common source with extremely high degree of certaintyExamples:Matching ridge characteristics of 2 fingerprintsComparison of random striation markings on bullets or tool marks (figure 3-1)Comparison of irregular & random wear patterns in tire or footwear impressions
25 Individual Characteristics Handwriting characteristicsFitting together of irregular edges of broken pieces like a jigsaw puzzleMatching sequentially made plastic bags by striation marks running across the bags (figure3-2)
26 Individual Characteristics Not possible to state with mathematical exactness probability that specimens are of common originConclusion must be made by practical experience of the examinerprobability of 2 individuals having same fingerprints is 1 x 1060
27 Individual Characteristics Individual characteristics can be used to distinguish members of the same classForensic scientists try to individualize a piece of physical evidence by some type of comparison processOnly a few types of physical evidence (primarily physical pattern evidence) can be truly individualized
28 The Ultimate GoalUltimate goal would be to move all class evidence to individualistic which is not very likelyHair is trying to make the move...something like paint probably never will.For human evidence to be individualistic, the odds of two people matching the same piece of evidence must be 1 in about 7.5 billion, which is the population of the earth.
29 Class vs. IndividualWhen does evidence cross the line that distinguishes class from individual evidence?This is debated and disagreed among many forensic scientists.How many striations are necessary to individualize a mark to a single tool and no other?How many color layers individualize a paint chip to a single car?How many ridge characteristics individualize a fingerprint?
30 Class vs. IndividualHow many handwriting characteristics tie a person to a signature?Up to forensic scientist to find as many characteristics as possible to compare one substance to another.Significant to consider:QualityComposition of evidenceCase historyExaminer’s experience
31 Functions of the Forensic Scientist Analysis of Physical Evidence1. Apply scientific techniques to analysis of evidence2. Be aware of demands and constraints of legal system
32 Functions of the Forensic Scientist B. Provision of expert testimony1. Expert witness – an individual whom the court determines possesses knowledge relevant to the trial that is not expected of the average person2. Competency shown by degrees held, professional organizations belong to, professional articles published, years of experience, participation in special courses
33 Expert Witness3. Opposing attorney may cross-examine the witness & point out weaknesses in background or knowledge4. Laypersons testifying may not state their opinion, but an expert witness can
34 Functions of the Forensic Scientist C. Furnishing Training in the Proper Recognition, Collection, & Preservation of Physical Evidence1. often have Evidence Technicians on 24 hour call to aid criminal investigations in retrieving evidence2. where patrol officers or detectives gather the evidence, forensic scientist should train all officers engaged in fieldwork