Principle of Forensic Science

Shivi Forensics
PRINCIPLES OF FORENSIC SCIENCE

Created by -
Sohini Ghosh 
Volunteer, Shivi Forensics 
Introduction
Forensic science applies scientific principles to legal matters, and its principles provide a framework for evidence analysis , from crime scene investigation to courtroom presentation. There are total seven core principles of forensic science.

The core principles of forensic science are:
1) Principle of individuality
    Statement: “Every object natural or man- made has an individuality which is not duplicated in any other object.”
    Explanation: The principles implies that no two objects or persons are exactly alike. Like , no two bullets fired from different guns have the same rifling marks. Forensic scientist use this uniqueness to match evidence like handwriting , tool mark etc.
   Example: A fingerprint found in a crime scene is compared with the suspects.
 If all rigid characteristics match the fingerprints can be uniquely attributed to thr suspect , proving there presence.

2) Principle of exchange ( Locard’s exchange principle )
     Statement: “Whenever two objects come into contact , there is always a transfer of material fromone to the other”.
     Explanation: This principle was proposed bu Dr. Edmond locard this is one of the most fundamental concepts of forensic science. It states that a criminal will always leave some thing behind at the scene of crime as an evidence and also take some thing away. This transfer may be in the form of hair, fibres , soil etc.
    Example: In a rape case the suspect will leave some cloth piece , semen , bitemarks etc . And will take away blood stains , hair , nail scratches etc of victim. This can be collected and examined in the forensic lab.

3) Principle of analysis
     Statement: “The analysis must be carried out using scientific methods and only after proper examinations”.
     Explanation: This principle emphasizes that all forensic evidence must be examined using scientifically accepted procedures and protocols . No guesswork or assumptions should be involved.
     Example: Blood samples collected from a crime scene are analysed using techniques such as ELISA or PCR , not by visual estimation or non-scientific methods. These analytical processes are standarised to ensure accuracy.

4) Principle of probability
    Statement: “ Forensic science works on the basis of probability and not on certainity”.
     Explanation: Forensic conclusions are often probabilistic. This is especially important in DNA profiling , trace evidence , handwriting analysis.
     Example: A DNA match may be stated as: “The probability of this DNA profile matching someone else from the population is 1 to 10 million” . This demonstrates the strength of the evidence without claiming absolute certainity.

5) Law of progressive change
    Statement: “Everything changes with time, and so does the condition of evidence.”
   Explanation: This principle refers to the fact that physical evidence is subject to change over time due to environmental factors , decomposition and human handling. Therefore, timely collection and proper preservation are critical to maintain its integrity.
    Example: Blood stains may degrade due to bacterial action or moisture. Therefore , forensic experts must collect and refrigerate biological evidence promptly to prevent spoilage.

6) Principle of comparison
   Statement: “The evidence found must be compared with known standards or control samples.”
   Explanation: No item of evidence can stand alone . Its forensic value comes only when compared to a reference or control.
   Example : A bullet recovered from a body is compared with a test bullet fired from the suspect’s gun if the rifling marks match.

7) Principle of Documentation ( Chain of Custody)
    Statement: “Proper documentation of evidence from the crime scene to the courtroom is essential.”
    Explanation: The value of evidence depends on its proper handling and traceability. The chain of custody ensures that the evidence was not tampered with and is admissible in the court. Each person who handled the evidence must be documented.
    Example: A sealed evidence bag with a fingerprint slide is signed and dated by the collector , lab analyst and presenting officer . If any step is missed the court must reject the evidence.

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