Evidence is defined as a means of determining the truth.
Physical evidence deals with material objects. It may be material left or taken from the scene of a crime by the suspect or victim, or it might be an impression left in some material. It includes not only footprints, but also hair, fibres, blood, or almost anything that can be deposited and collected. Unlike oral testimony, it is not influenced by the stress of the moment; it does not forget. Physical evidence can aid in solving the case by developing an overview of events, By identifying animals, by proving or disproving events and by providing investigative leads. Physical evidence is often necessary to prove that an event has occurred.
The amount of consideration given to physical evidence depends on whether the evidence has individual or class characteristics. Evidence with individual characteristics can definitely be identified with an animal or source if sufficient microscopic or identifiable markers are present.
Evidence with class characteristics, no matter how thoroughly examined, will only be placed in a class or group. A definite identification can never be made. There is always a possibility of more than one source for the material found. Some examples are hair, fibres, blood, and soil. It is, of course, desirable to have evidence that can be positively identified, but cases can be made on evidence with class characteristics only. This type of evidence can help build the case of circumstantial evidence or prove an event false. Just as important, this type of evidence can give a definite negative, i.e., proving positively that a particular piece of evidence did not come from a particular source.
A better case can be made with class evidence when the evidence has either a greater number of identifying features or a greater number of different types of evidence. Identifying features include soil with foreign matter.
Since UPIA have more case work than analytical time, the submitter can aid the examiner by fully relaying the facts of the case. Information given to the UPIA will establish the direction of the analysis and may help to determine the worthiness of the evidence. Many examinations are lengthy and expensive. The efficiency of the analysis provided is directly related to keeping the analysts well informed as to the facts of the case.