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Forensic Study: The Importance of Forensics to Court Cases: The importance of forensic science to law lies in its potential to provide evidence of a highly specialised nature. It helps to establish who committed a crime, and how. It is important to find when the time of death occurred, but this is almost impossible. Forensic science serves the law. The law is not concerned with developing scientific theory or methodology. The subject matter of forensic science is the law, and it is the job of the law to scrutinise the science. Over the years the law has impacted on forensic science so as to (a) Help develop scientific theory and methodology. (b) Eliminate all subjectivity from forensic evidence. (c) Ensure that what is proffered as admissible forensic evidence equates with credible scientic thinking, methods and conclusions. (d) Try and eliminate non-scientific evidence. Scientific knowledge can change over time. Class Characteristics:
- General nature
- Shared by a group and not a single source e.g. Tyres, fibres, glass, firearms, gunshot residue, hair, soil, chemical substances. They can provide corroborative and contextual evidence. "Class characteristics" refers (a) To a specific person or an object with more than one identifiable feature. (b) Only to people. (c) To physical evidence that can only be related to a similar type or group of evidence and not a single source. (d) Does not refer to any of the above. Individual Characteristics: These are characteristics which can be directly associated with a person or object. e.g. DNA, fingerprints, dental records, ear form, matching rifle marks on a bullet. Of the trace evidence found at a murder scene which of the following pairs of evidence can best provide individual characteristics?
(a) Patterns of blood spatter and shattered glass. (b) Car tyre imprints and paint chips. (c) Fingerprints and semen. (d) Silk fibres and weapon marks on a body. Crime Scene Management: Very important. Crime scene management is the scrupulous recording of where a crime took place and the careful collection of physical evidence. May involve sealing off a crime scene for later investigation. It is the single most important job of the police, the forensic team, or other investigators. It is destructive when you look at a crime scene - nothing is the same again. When practical, the crime scene should be sealed off, especially if it looks like an arrest will be
Establish one path for moving in and out (minimise any disturbance of the scene) Find an inert staging area for storing tools and protective clothing Photograph as you go. Note the environment, grasses, soils, areas of possible disturbance. Look for what might be out of place.
Before any excavation takes place, search the entire scene of crime (SOC). Often people will have been interviewed. Look for evidence that matches the witnesses statement, or evidence that contradicts it. Flag and document potential evidence. Proceed in a methodical way. As a lawyer, you look at the methodology. The most important procedure used in crime scene management is (a) Identifying the crime scene and maintaining its integrity against the contamination of evidence. (b) Photographing and sketching the crime scene. (c) Faithfully documenting and recording all finds of evidence for any possible later use in court. (d) All of the above are just as important as each other. Importance of dead and living insects - may indicate the type of food they eat. The number and state of development of insects may indicate a time of burial. It is often possible to calculate a time span since death based on the numbers of dead generations of insects. Body Cavities:
* Beware of rats
* Beware of syringes
* Beware of knives, broken glass
* Never put your hand into a cavity without seeing where it is going After the body has been removed, continue excavating the soil below, removing soil samples any any other material that could be used as evidence. Basic Procedures:
Secure and isolate the crime scene
Record the scene
Record what personnel did
Search systematically for evidence
Secure the chain of custody of evidence
Undertake crime scene reconstruction Need to understand all the steps, so you can see if anything went wrong. Crime scene reconstruction has a high degree of subjectivity. One danger is that it can lead to tunnel vision - you end up fitting the evidence to a person, and become fixated. There is a danger of slipping into why you think something has occurred, instead of trying to determine what has occurred. In the absence of a confession, you must be careful about coming to conclusions as to motives. Direct Evidence: Was the offender photographed committing the act in broad daylight? Was the
DNA profile identical to the profile of the suspect?
Circumstantial Evidence: Contributes to ID but can often be interpreted in different ways. Peripheral Evidence: Highly questionable. May not even resemble evidence at all. Creates a distraction and is liable to distort the presentation of evidence, juries can be sucked in by it. Solution: try to get rid of peripheral evidence asap. Discovery: When you are the defence, and you are dealing with evidence, you must ask. Get into a good working relationship with a forensic scientist.
Never accept still images. Never accept anything less than an unaltered video copy. Police notebooks and plans. Do the police notebooks corroborate what the prosecution is saying?
DNA lab notes
Probability and Likelihood: Basic rule: If A or B, add the probabilities. If A and B, multiply the probabilities. Odds describe probability. Odds tell us how much more likely A is than B. Probability: A = event (it is raining)
~A = negative of event (it is not raining) P(A or ~A) = 1 Odds: If we assume B = ~A, and if A = 25%, then B = 75%, the odds are A/B, 25/75 or 1:3. To convert odds to probability, the calculation is (odds in favour)/(odds in favour + odds against) (e.g. 1/(1+3) = 1/4, = 25%) Coin Toss: P(head) = 1/2
P(head or tail) = P(H or ~H), 1/2 + 1/2 = 100%
P(head and head) = P(HxH), 1/2 x 1/2 = 25%
Weighted Coin: P(head) = P(fair coin x heads), 1/2 x 1/2, + P(biased coin x heads), 1/2 x 4/5, = 1/4 + 2/5 = 65%
Bridge Hand: Each bridge hand contains 13/52 cards. P(bridge hand containing ace of spades) = 1/4
P(bridge hand containing ace and king of spades) = 13/52 x 12/51 = 5.88%
Although the probability of a particular hand is infintesimally small, each hand has exactly the same odds. Likelihood Ratio: Ratio of the two probabilities for the same assertion using different conditions. P(E/C)/P(E/~C) P(E/C) = the probability that the defendant's DNA profile would be found to match the crime sample profile given that the defendant is the source of the DNA in the crime sample. P(E/~C) = the probability that the defendant's DNA profile would be found to match the crime
sample profile given that the defendant is not the source of the DNA in the crime sample. There are always two hypotheses - Defendant committed the crime (D) and Defendant did not commit the crime (~D). The difference between likelihood ratio and odds: Likelihood ratio is the ratio of the two probabilities for the same assertion using different conditions. Odds is the ratio for alternative assertions under the same conditions. Bayes' Rule: The value of a piece of evidence in testing a particular assertion against an alternative is determined by its likelihood ratio. Prior odds x likelihood ratio = posterior odds If the prior odds are 1, then the likelihood ratio becomes the posterior odds The evidence must be independent without double counting i.e. odds of blood group and DNA could only be considered if they were not dependant on each other Logic Logical proposition - All men wear hats. John is a man. John wears a hat. Illogical proposition - All men wear hats. John wears a hat. John is a man. Need to ensure propositions are exclusive in assessing probabilities: 80% of abused children bite nails - of no value in assessing whether a child is abused, unless you know how many non-abused children bite nails. Logically correct, but no evidential value - if x is abused, then there is an 80% chance will bite nails. Logically incorrect, and likely to mislead - if x bites nails, then 80% chance abused. If 80% of abused children bite nails, and 10% non abused children bite nails, then the propositions are exclusive: The result is that an abused child is 8 times more likely to bite nails than a nonabused child. Prosecutors' Fallacy: Occurs when the conditional is transposed, so that P(E/H) is the same as P(H/E). The probability that an animal has four legs if it is a cow is 100%. Fallacy - The probability that an animal is a cow if it has four legs is 100%. The Birmingham 6 case: Evidence was 99% certain that clothes contained explosive residues as the use of explosives left such residues 99% of the time. Flaw was that they did not consider what percentage of persons would have the residue if they had not handled the explosives. R v Cannan: "So far as the DNA was concerned it seems that the chances of anyone else having been responsible for this semes was something like 260 million to one against". In fact the evidence would have been the characteristics were shared by 1 in 260 million and the odds of a match by chance have been transposed to odds in favour of guilt. Defence Fallacy: One in 48,000 people have DNA profile. Fallacy - There are 72 other people in NZ who could have left the DNA. It is a fallacy because it overstates the position by ignoring other associated evidence, as a number of other people would have been excluded by age, gender, location etc. Preece v HM Advocate: Stains at the murder were group A (secretor) which was the accused's blood
group. The evidence of the probability that it was the Accused's rather than by chance was given even though the deceased was also group A, and it may not have been possible to exclude the blood of being that of the deceased. The Importance of Trace Evidence: There is subjectivity in all scientific work, because if you make any decisions about anything, you affect the result. Scientists try to limit the amount of subjectivity by having a rigorous methodology. Trace Evidence: Tangible, physical evidence. It is usually found in small quantities, and its analysis often involves the use of a variety of specialised instrumentation such as high and low power microscopes. Trace evidence analysis is a necessary part of the laboratory process carried out under strictly controlled conditions. e.g. hair, fibres, paint, glass, tyre prints, tool marks, soil, gunpowder residue, pollen, insects. The nature of trace evidence is typically a class characteristic. Sometimes, however, evidence such as DNA or fingerprints are obtained (individual characteristics) and point directly to the offender of a crime. From collection to presentation in court, trace evidence must be bound by an unbroken chain of custody. Hair - Trace Evidence: To identify someone from a strand of hair you have to get the follicle as well; the follicle contains the DNA. The first step in the analysis of hair is to determine whether or not it is human. It's the same with blood - Is it blood? Is it human blood? Who's blood is it?
You can tell whether hair has been dyed or bleached, and can look at hair length, diameter, and compare it with a hair library or with hair from one's own body. Unless you have the follicle, you can not tell the sex of an individual from their hair.
Seldom reveals who committed an offence May not indicate when an offence was committed May indicate how it got to where it was found May not have been analysed by a competent person
Witness Examination: The purpose of examining a witness is two-fold:
- To get the evidence supporting your client's case before the court; and
- To have the Judge or jury accept what the witness says is the truth Evidence must be presented in a persuasive manner s83 Evidence Act Ordinary Way of Giving Evidence. Leading Questions - A question that directly or indirectly suggests a particular answer to the question. S89 Evidence Act Leading Suggestions in Examination in Chief and Re-Examination
- Not allowed leading questions unless - the question relates to introductory or undisputed matters; or, the question is put with the consent of all other parties; or the judge allows the question.
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