FORENSIC EVIDENCE

Evidence:

From Wikipedia, the free encyclopedia

Evidence in its broadest sense, includes anything that is used to determine or demonstrate the truth of an assertion. Philosophically, evidence can include propositions which are presumed to be true used in support of other propositions that are presumed to be falsifiable. The term has specialized meanings when used with respect to specific fields, such as policy, scientific research, criminal investigations, and legal discourse.

Postmodern thought, such as research undertaken within the philosophy department of University of North Carolina, Chapel Hill, theorises evidence as a commodity. The most immediate form of evidence available to an individual is the observations of that person's own senses. For example an observer wishing for evidence that the sky is blue need only look at the sky. However this same example illustrates some of the difficulties of evidence as well:

  • someone who was blue-yellow color blind, but did not know it, would have a very different perception of what color the sky was than someone who was not. Even simple sensory perceptions (qualia) ultimately are subjective; guaranteeing that the same information can be considered somehow true in an objective sense is the main challenge of establishing standards of evidence.
  • there is also the question of what is meant by 'blue', and how we measure it. (If determined by a particular wave-length of colour - then how do we actually measure this?)
  • there is also the question of how evidence 'translates' e.g. is 'blau' in German universally translated as 'blue' in English: Germans may have different words for different parts of the spectrum; thus 'evidence' is a social construction.

Evidence in science

Main article: Scientific evidence

In scientific research evidence is accumulated through observations of phenomena that occur in the natural world, or which are created as experiments in a laboratory. Scientific evidence usually goes towards supporting or rejecting a hypothesis. When evidence is contradictory to theoretical expectations, the evidence and the ways of making it are often closely scrutinized (see experimenter's regress) and only at the end of this process the theory is rejected: in that case we call that refutation of the theory. The rules for evidence used by science are collected systematically in an attempt to avoid the bias inherent to anecdotal evidence: nonetheless even one anecdotal evidence when possible to replicate is enough to reject a theory incompatible with that evidence.

Evidence in criminal investigation

In criminal investigation, rather than attempting to prove an abstract or hypothetical point, the evidence gatherers are attempting to determine who is responsible for a criminal act. The focus of criminal evidence is to connect physical evidence and reports of witnesses to a specific person.

Evidence in law

Main article: Evidence (law)

Legal evidence differs from the above in the tight rules governing the presentation of facts that tend to prove or disprove the point at issue. In law, certain policies require that evidence that tends to prove or disprove an assertion or fact must nevertheless be excluded from consideration based either on indicia relating to reliability, or on broader social concerns. Testimony (which tells) and exhibits (which show) are the two main categories of evidence presented at a trial or hearing.

Evidence in statistics

This is the key of statistical inference, for which see e.g. the work of Allan Birnbaum and others.

Types of evidence

External links


Trace evidence:

Trace evidence is material found at a crime scene or accident scene in small but measurable amounts. This is important as it can definitively link an individual or object to the scene. Trace evidence is based on Locard's exchange principle which contends that every contact no matter how slight will leave a trace. The trace is normally caused by objects or substances contacting one another, and leaving a minute sample on the contact surfaces. Material is often transferred by heat induced by contact friction.

Examples:

Examples of typical trace evidence in criminal cases include hairs, fibers, glass, paint chips, soils, botanical materials, gunshot residue, explosives residue, and volatile hydrocarbons (arson evidence). For such evidence to be useful, it must be compared to similar items from suspects, but particular care is necessary to ensure a thorough analysis.

Ladder feet often leave a trace pattern on the ground, so showing how the ladder moved and caused an accident to the user. Skid marks from tires are often critical in determining the sequence of events before and during a car crash. Vehicular accident reconstruction relies on such marks to estimate vehicle speed before and during an accident, as well as braking and impact forces.

Collection

Many different techniques are used in the collection of trace evidence including shaking, brushing, taping, vacuuming, swabbing and hand picking. In some cases, such as with oil or grease, a solvent extraction can be used to collect the evidence for analysis. The method used for collection is generally dependent on both the type of evidence and from where or what sort of object it is being collected from.

Analysis

Droplets of human blood. In addition to analyzing for DNA, the droplets are round and show no splattering, indicating they impacted at a relatively slow velocity, in this case from a height of two feet.

 

Droplets of human blood. In addition to analyzing for DNA, the droplets are round and show no splattering, indicating they impacted at a relatively slow velocity, in this case from a height of two feet.

Analysis of trace materials most often begins with a visual examination of the evidence. This is then usually followed by microscopic analysis, of which a number of different types are available dependent on the type of material to be analysed such as a stereomicroscope, scanning electron microscope or comparison microscope.

Gunshot residue may be identified by elemental analysis using atomic absorption or with a scanning electron microscope equipped with an energy dispersive spectroscope. Small amounts of explosives, volatile hydrocarbons, and other chemicals are identified with the use of analytical instruments, such as gas chromatography, mass spectrometry, and infrared spectroscopy.

Similar comments apply to damaged items from an accident scene, but care is needed in ensuring that the sample is not damaged by the testing, or sampling for testing. Such nondestructive testing must always be used first before then considering methods which involve taking small samples from the item for more detailed tests, such as spectroscopic analysis. Use of all such methods must be done in consultation with other experts and the relevant authorities, such as lawyers on both sides of a case.

Problems

False positives and contamination are frequent problems owing to the presence of many common substances and the necessity of human involvement in the collection of trace evidence.


Transient evidence:

Transient evidence is evidence that has no meaning or is so false it is easy to see through it. Transient evidence may be lost a short period of time after the crime took place. It must be identified and recorded immediately upon investigation. If possible, it should be protected or preserved for later review. For example, weather and witness recollection are considered transient evidence.


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