Introduction to Forensic Science
Lesson 4
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Lesson 4 Analysis of Evidence
Lesson Aim
Explain how different types of evidence are tested and analysed on site and in the laboratory.
Discuss how evidence may be used and/or interpreted.
INTRODUCTION
Evidence is information which is used to prosecute or exonerate a defendant, plaintiff, or other party in a legal setting. It may be used to link an individual to a crime scene, other place, or set of actions. This means it is collected using strict protocols. If evidence has not been collected according to proper, previously defined protocols, then it generally cannot be used in court. It is important to note that the protocols governing evidence collection, use, and admission vary according to governments and geographic region. Such laws and protocols can vary across council or county lines, state lines, international lines, and more.
It must be noted that trace evidence refers to small, minute, or microscopic evidence. A suit jacket left at the scene of a crime is evidence, not trace evidence. The small blood spatters on the suit jacket are trace evidence.
All evidence must be evaluated properly if it is to be admissible in court. A case can be withdrawn from court if the evidence is not accepted.
THE PURPOSE OF ANALYSIS
Analysis is a key part of the investigative process. It is dependent on the proper use of the scientific method, chain of custody, and objective interpretation. The purpose of analysis is to link a given thing or person to a person, place, object, or more. Analysis of evidence helps investigators create a narrative. If the evidence and analysis do not fit the narrative, then the narrative must be changed. It is essential to understand this chain: the evidence creates the narrative. The narrative does not create the evidence.
When analysing and interpreting evidence, it is essential that all evidence be included in deliberations. Forensic professionals cannot cherrypick the evidence they wish to interpret. In some cases, evidence may be excluded from later analysis due to errors in collection or reasonable exclusions (determined by a plethora of investigate and legal rules beyond the scope of this course).
PRINCIPLES UNDERLYING ANALYSIS
Forensic identification is based on 7 main principles:
1) Individuality principle
The principle of individuality, attributed to Paul L Kirk (1963), is regarded as the building block of forensic science. Individuality implies that every entity, whether person or object, can only be identical to itself and so is unique. No two objects whether natural or artificial can be exactly the same. Kirk claimed that the aim of forensic science is to focus on the source of two items (questioned and known, or mark and print), which are thought to have come from a single source.
As such, identification is concerned with establishing individuality from traces left at a crime scene rather than the sameness of two things. This means identification can be shown indirectly through the analysis of traces and samples e.g. no two fingerprints are the same.
2) Exchange principle
The exchange principle is attributed to Edmond Locard. The principle states that whenever two objects or subjects interact, some sort of trace will be left behind. This principle underpins the search for trace evidence at crime scenes. Trace materials include hairs, blood, fibres, and gunshot residue.
Locard suggested that there are many traces left behind and if interpreted properly, they can provide valuable information.
There are several other important general principles underlying the execution of science for forensic purposes, including:
3) Law of progressive change
Different objects change, although they may change across different time spans. For example, blood samples will eventually degrade. Some objects are more durable than others and may be relatively permanent, remaining mostly unchanged during identification. If an object is very durable it may be quite easy use it for identification. If it is less permanent and its main features change during the identification process it may have less pragmatic use, particularly in time-sensitive environments.
4) Law of comparison
Different samples must only be compared to samples which are alike. In other words, blood samples are
compared to other blood samples, fibres are compared to other fibres, and so forth. In common parlance, this is sometimes referred to as “apples to apples comparison”.
5) The Law of analysis
The quality of any analysis is determined by the quality of the sample under analysis, the chain of custody, and the expertise of the individual who analyses it.
6) The Law of circumstantial facts
This is concerned with eyewitness testimony, victim statements, and so forth. Anytime that people are called upon to provide evidence there is a chance that the evidence they supply is inaccurate. This can be
unintentional e.g. through mistaken observations, making assumptions or deliberate actions e.g. lying or exaggerating. On the contrary, evidence which gives a factual account e.g. based on investigation and evidence has a higher chance of being accurate and is more reliable.
7) The Law of probability
Conclusions drawn from forensic analysis are dependent on the method used and its advantages and
disadvantages. All of these factors must be considered for a reasonable, relevant, and admissible conclusion to be drawn.
Forensic analysis depends on both the discovery of trace evidence then connecting the pieces to individuals, locations, and more. If there are no traces found at a crime scene, it may be impossible to identify linked persons or localities. If traces are found and appropriate analysis undertaken, they may be used as evidence.
How these laws apply to analysis
Each of these laws is important in understanding how and why we analyse evidence in specific ways. One law cannot be used to override another, or make an argument for making the facts fit an existing narrative. These seven known laws are the basis for how we reasonably draw conclusions based on objective facts (as opposed to subjective opinions).
Analyses must be interpreted by an expert. This is different to interpretation by a layperson. Laypersons may have a set of reasonable and standard knowledge about an idea, concept, or field. Experts have specialist knowledge derived from extensive, relevant learning used in conjunction with practical applications. When interpreting evidence, expert findings may differ. When this happens, multiple experts are often consulted, and decisions made according to weight of opinions, exclusion of possible biases, and when possible, retesting.
STATISTICAL ANALYSES
In recent decades, the statistical evaluation of evidence has been under review and development. Given the increase in use of statistical evidence evaluation there is a need for reliable and representative data to underpin evaluations.
Many different types of statistical analysis have been performed on evidence in the past, but these days it is evidence is usually interpreted using a ‘likelihood ratio’. A likelihood ratio is said to be supported by Bayesian reasoning, which is considered to be the right way for making decisions where there is a degree of uncertainty.
It is based on the ratio of the probability of a claim being true compared to the probability of it being false.
According to Bayes rule, in the light of new evidence individuals multiply prior odds by an appropriate likelihood ratio to arrive at posterior, or updated, odds which show their new degree of belief in a claim being true.
A key component of statistical analysis is the possibility of objective enquiry and decision-making. In assigning end analysis to a computer or machine-learning, the potential for human bias is removed. However, there are three major risks to statistical analyses of evidence: human/user error; correlation vs causation; misused univariate analysis.
Human/User Error
Humans are fallible. When we enter material into a database or other program, the program is dependent on correct entry. Similarly, the analysis run – at the click of a proverbial button – must be correct. If a human does not use the software correctly, the software will not work correctly, and evidence will be corrupted.
Included in this is the choice of data used. If there is a human bias toward an action – say, arresting people of a certain race at higher rates due to human bias – the bias will carry into the system, even with correct entry. For this reason, it is often important to use multiple sources and larger sample sizes for determining trends relevant to a potential investigation. It is important to note that while such work is rarely necessary for crimes such as assault or murder, it may be necessary for financial, art, and other theft.
Correlation vs Causation
A correlation between two things is a link. A causation is what it sounds like, a cause.
Let’s say we’re studying dogs. We learn through our research that 66% of dogs wag their tails on Tuesdays, while 56% wag their tails on Mondays. We can see that there is a link between higher tail wagging and the day Tuesday. This does not mean that all dogs wag their tails more on Tuesdays because it is Tuesday. It simply means that we see more tail wagging on Tuesday.
Once we see a correlation, we can go looking for causation. If we find that all factors save one in our sample are the same, we can look at the outstanding factor as a possible cause. In this case, we find that:
All owners come home at the same time everyday
All dogs are the same breed
All dogs are in the same age range
All dogs are male
The postman comes only on Tuesdays
This now shows us there is a correlation between the postman’s day and dog tail wagging increasing. But without further research, we cannot definitively state that the postman is the cause of the increased tail wagging.
Statistical analysis of evidence requires us to understand the difference between correlation and causation, and narrow down other factors. It may also require us to return to our evidence and look for new information, or run extra tests or studies, such as recording dog interactions.
Univariate Analysis
This is when a single variable is used during analysis. While univariate analysis does have its place, most statistical analysis of evidence requires acknowledging and controlling for multiple variables, including factors such as socioeconomic status, family structure, education level, sex, gender, and more.
TYPES OF EVIDENCE
Most of the evidence discussed in this course has been trace evidence, such as blood spatter and DNA. There are, however, many kinds of evidence, and what is relevant will depend on the situation in question. Some crimes or situations use bank statements, provenance, proofs of authenticity and more as evidence. These also require detailed forensic analysis by specialists to help determine patterns, usage, and more.
Laboratory Analysis
When evidence has been collected from a crime scene investigation it is usually taken to a laboratory for analysis. The forensic laboratory may process all of the evidence found at the crime scene or just some of it depending on the types of evidence recovered. Some may have to be processed by other more specialist services. It is more usual for individual technicians based at laboratories to be specialists in particular types of analyses or evidence classes rather than conducting a wide variety of tests on different sorts of evidence.
Some examples of laboratory analysis of evidence include:
hair
fingerprints
DNA
blood
drugs and toxicology
Blood Spatter
Blood spatter analysis draws on the patterns of blood spatter observed at the crime scene. Blood traces can be found in the surrounding area, on clothing, on objects, and more. To complete this type of analysis, experts consider the likely pattern, though it is worth noting that many blood spatter analysts are in law enforcement and are not forensic pathologists. In recent years, verdicts depending on blood spatter analysis have been overturned in the US; some non-profits are currently raising questions about training, accreditation, and more.
Hair
In most cases, the natural human body is covered in hair. There are different types of hair in different zones of the body. Hair also has several stages in its growth.
The structure of hair is determined by its keratin. Keratin is a type of protein, and the basic foundation for hair, fingernails, claws, feathers, and more. There are two types of keratin present in hair. The amount of each type and how they are linked to one another affects the shape and thickness of the hair.
When a root is present, hair can also be used for DNA analysis. A hair shaft, without the follicle (also known as the root), can only be analysed for thickness, type, keratin structure, natural colour, and dye colour.
Fingerprints
Fingerprints are ridges on the skin, on the fingertip. Several places on the human body have such ridges, sometimes known as epidermal ridges. Fingerprints are formed in utero. Although each human has a distinct fingerprint, the systems used for comparing fingerprints are fallible, meaning that while it is unlikely, it is possible to link lifted fingerprints to the wrong person.
Fingerprints deliberately collected via ink are usually called exemplar prints. Latent prints, or those accidentally left behind, may be left in a substance (e.g. ink or blood) or as a small amount of oil on a surface. While these are useful in evidence collection and analysis, they may be smudged or unclear.
Patent prints are ridges left as an impression in another substance, such as clay or playdough. These are created by the actual shape of the finger interacting with a material, as opposed to an oil or other liquid deposit.
Fingerprint Matching, or Dactyloscopy
It is important to note that this is a simple overview of a complex process.
While there are several different fingerprint systems available for analysis, the Henry system underpins the most widely used. In this system, fingerprints have a loop, whorl, or arch structure. From here, further notations are made for which finger and the occurrence of whorls. A computer or human analyst studies the structure of the print and marks whorls present in specific areas. These are compared to the prints in question.
DNA
DNA, or deoxyribose nucleic acid, makes up the human genome. This is how genetic information is carried across almost all living organisms. All cells are derived from pre-existing cells and are formed through a process of cell division. As a cell divides all new cells need to have a copy of the genetic material carried on DNA which means that DNA must be replicated before cell division.
DNA analysis lets technicians look at the makeup of the DNA. DNA can be extracted from blood, skin cells (touch DNA, or on a victim), hair follicles, and other tissues. Reference swabs, or comparison swabs, are usually taken from the inside of the cheek (buccal swabs).
There are different types of analysis. The first step is usually to chop the DNA into fragments using gel electrophoresis. These fragments are then analysed to identify base pair sequencing, creating a type of DNA fingerprint. This allows technicians to compare reference samples and evidence samples.
In some cases, mitochondrial DNA is used. This may be because there isn’t enough cellular DNA available, or to help link the sample to maternal relatives.
Touch DNA
This is when a very small amount of cellular material – with DNA – is left at a scene. Humans shed skin cells from the uppermost layers of skin as they move throughout the world. Touch DNA is left behind when something is brushed or lightly touched. It comes from the uppermost layers of skin cells.
Touch DNA analysis uses a very small number of skin cells – analysis with fewer than 10 cells is possible. This means the risk of contamination is high, and false positives are more likely to occur.
Low Copy Number DNA
Also known as LCN DNA analysis, this is when a sample is run through additional cycles of polymerase chain reaction amplification.
LCN DNA analysis allows scientists to analyse DNA from a very small number of cells. This is distinct from analysis of touch DNA – touch DNA does not use extra cycles of PCR. However, LCN analysis has been criticised due to the high risk of contamination. If a sample is contaminated before amplification, it will affect the final result. Moreover, the extra cycles of amplification can create artefacts within the final analysis, much like photocopying a photocopy repeatedly leads to artefacts in the final result.
The use of LCN DNA analysis was suspended in the UK due to criticism regarding the lack of wider scientific validation and potential contamination issues. At the time of this writing, the suspension has been lifted.
Blood
In most cases, blood is used in two ways: pattern/spatter analysis and typing.
Spatter analysis looks for patterns in blood around a scene. It usually also accounts for the amount of blood found. This type of analysis helps technicians determine the wound type, where an attack or incident took place, and even weapon used. Technicians document blood patterns around a scene with markers and photographs. These are used to enter blood evidence into evidence. Blood evidence may also include prints, such as fingerprints, palm prints, or shoe prints from contact with the blood.
Blood type analysis looks at blood type and rhesus factor. Blood types come in O, A, B, and AB, with an associated rhesus factor (+ or -). Blood type analysis works by looking at the antigens in the blood. This is
combined with a positive or negative. The most common blood type, according to the Red Cross, is O+, followed by A+, B+, then O-. AB- is the least common blood type.
Blood may also be used for DNA analysis.
Drugs & Toxicology
This type of analysis looks at chemicals present in the body. It usually looks for the presence of drugs and other chemical types in blood and tissues. There are different levels of screening. The most common is a screen for alcohol and common drugs, such as illegal street drugs and commonly prescribed drugs. More detailed screenings may look for the presence of gases, poisons, and unusual levels of metals and heavy metals.
USE OF LABORATORY EVIDENCE
After evidence has been tested, the results are sent to the lead detectives investigating the case. The test results eventually lead to a reconstruction of the crime scene.
In other words, a hypothesis can be formulated about the order of events from before the crime was committed until it happened and just afterwards. Detectives review the evidence supplied by the laboratory and try to work out how it all fits together in the criminal act. Evidence is also compared to witness statements to see how reliable their accounts are.
Evidence may then be used to link victims or suspects to crime scenes, identify a victim or suspect, confirm witness statements, and so forth.
SET TASKS
Set Task 1
Select three types of substances screened for in drug and toxicology testing. Research screening techniques for these substances. Spend no more than 1 hour on this task.
Set Task 2
Research the Law of Circumstantial Analysis and circumstantial evidence. Spend no more than 1 hour on this task.
Set Task 3
Research types of DNA analysis. Select 2 types, and read about this in depth. Spend no more than 1 hour on this task.
