1. Physical Evidence Common types of Physical evidence Identification of comparison of Physical evidence Individual and class characteristics Class evidence and it is value in criminal investigations Databases available for Forensic scientists

2. Common Evidence Items Found in Crime Scenes Blood, Semen and Saliva – all suspected samples Documents – written, type written, indented, obliterated and burned and charred Drugs Explosives Fibers Finger prints Firearms and ammunition Glass Hair Impressions Organs physiological fluids Paint Petroleum products Plastic bags, plastic and rubber and other polymers Powder residues Serial numbers Soils and minerals Tool marks Vehicle lights Wood and other vegetative matter

3. Physical Evidence Common types of Physical evidence Identification of comparison of Physical evidence Individual and class characteristics Class evidence and it is value in criminal investigations Databases available for Forensic scientists

4. The examination of physical evidence is undertaken for identification or comparison Identification: the process of determining a substance’s physical or chemical identity. –Example: Drug analysis, species determination, and explosive residue analysis are typical examples of this undertaking in a forensic setting Comparison: the process of ascertaining whether two or more objects have a common origin –Example side-by-side comparison of hair from a suspect vs hair found in the crime scene Physical Evidence Hair found in Crime Scene Hair from the suspect

5. Physical Evidence Common types of Physical evidence Identification of comparison of Physical evidence Individual and class characteristics Class evidence and it is value in criminal investigations Databases available for Forensic scientists

6. Physical Evidence Individual characteristics: properties of evidence that can be attributed to a common source with an extremely high degree of certainty Class characteristics: properties of evidence that can only be associated with a group and never with a single source –Plastic bags –Blood group comparisons Piece of Knife found in the stab wound in the neck of victim Tip of Knife found pocket of suspect Plastic bag found in possession of a suspect Plastic bag found covered on the victim’s head

7. Physical Evidence Common types of Physical evidence Identification of comparison of Physical evidence Individual and class characteristics Class evidence and it is value in criminal investigations Databases available for Forensic scientists

8. The Role of Forensic Science Provides scientific information to the legal community. Done via analysis of evidence, submission of reports and testimony. A statement of the limitations on conclusions drawn from an experimental result is an essential element in crime solving. Class Evidence and it is value in Criminal investigations

9. Limitations Analyst examines the chemical & physical traits of an evidence fiber These traits are similar to fibers from a reference carpet in the suspect’s house. Thousands of yards of that carpet are installed in homes across the world The evidence fiber is consistent with the reference carpet and the other thousands of carpets The amount of information that can be derived about the sources of the carpet is limited  by the type of examination performed  the nature of the evidence Assessing the Significance of Physical Evidence Side-by-Side Comparison of Fibers

10. Assessing the Value of Physical Evidence Responsibilities of the scientist May conclude that an evidence object & a reference object show similar traits Must understand and communicate the limitations inherent in the conclusion –limitations are a combined function of the nature of the traits the sensitivity and resolution of the detection methods the state of the evidence Limitations The more common the observed traits, the less significant is the similarity Insensitivity of detection method –inability to observe a difference that exists Deteriorated state of sample –obliteration of a difference that actually existed The strength of the evidence is an inverse function of its limitations

11. Questions Setting Limitations What traits are being examined? –Not well defined/thought-out What distinguishes this item from others in the same class? –Possibilities not explored How many different kinds of traits exist & which of these have been examined? –Too few traits considered for examination How much variability might exist within a single individual? –Unwilling to examine in depth How much variability exists in the population? –All possibilities not considered. SEM of a Paint Chip 240 X Magnification 1600 X Magnification

12. Legal vs. Scientific Questions The law must establish that a crime has been committed by defining the corpus delecti (body of the crime) All elements that legally define any particular crime must be present to proceed The forensic scientist must translate the legal question into a scientific question What crime was committed? How was the crime committed? “Who has done it?” Where is the true “scene of the crime”? When was the crime committed in time? Questions Science May Answer Question Science Cannot Answer Why was the crime committed?

13. The Framework Determine the legally relevant question Determine the assumptions Analyze the evidence to provide facts about the question Make an inference based on the assumptions & facts Fundamental Principles Transfer –Locard Exchange Principle Identification –placing objects in a class Individualization –narrowing the class to one

14. Forensic Databases The ultimate contribution of physical evidence analysis is to link the a suspect to a crime thorough comparative analysis. Forensic Databases are valuable tools in making such links –Fingerprint databases –DNA data bases –Nationally integrated ballistics network –International automotive paint data quiry –Iris Scans Databases

15. Fingerprint databases Every country and state has its own fingerprint databases Best known ones: –IAFIS-integrated automated fingerprint identification system (USA) –Computerized fingerprint database (UK)

16. DNA Databases Combined DNA index system (CODIS; FBI 1998) Rapid DNA data base (all arrestees – USA) National DNA database (UK, 1995) National DNA data bank (Canada) –These data bases hold – Restriction digestion maps –STRs – short tandem repeats –VNTR – variable number tandem repeats –Mitochondrial DNA databases

17. Other Databases Foot ware data bases Fibers directory Glass repositories Drug inquiry databases