Learn about the Principles that Underpin Forensic Analysis
Forensic identification is based two main principles i.e. individuality and exchange.
The Individuality Principle
principle of individuality as attributed to Paul L Kirk (1963) and is
regarded as the building block for 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
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 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 is generally at the crime scene. Trace materials include hairs,
blood, fibres, and gunshot residues.
Locard suggested that there are many traces left behind and if interpreted properly they provide the most valuable information.
There are some other more general principles which apply to forensic science:
The Law of Progressive Change
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 is not
possible to answer the question of sameness.
The Law of Comparison
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.
The Law of Analysis
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.
The Law of Circumstantial Facts
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 not accurate. This can be
unintentional e.g. through mistaken observations, making assumptions or
deliberate 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.
Law of Probability
drawn from forensic analysis are dependent on the method used and its
advantages and disadvantages. This all has to be taken into
In conclusion, forensic analysis depends
on both the discovery of traces, and connecting them to individuals. If
there are no traces found at a crime scene, it is impossible to identify
suspects. If traces are found then provided these are analysed properly
and the results interpreted in a suitable manner, they may be used as
WHO CAN BENEFIT FROM THIS COURSE?
- A crime writer wishing to produce more authentic fiction.
- Anyone working in a legal office seeking to better understand aspects of their work
- Security guards learning to be more observant of the property they guard.
- Investigative journalists seeking to expand their understanding of criminal activity.
- Anyone considering a career in law enforcement or criminal law; to
develop a fundamental understanding of the nature and scope of this
subject prior to deciding on pursuing more in depth studies.
- Anyone else with a passion, or need to understand more about forensic science.