Types of Evidence, and where to LOOK for them!

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Given that Forensics = Science & the Law, it is useful to refer to an earlier time, when witchcraft was a common crime (or rather the perception of witchcraft):

Let's examine, for a moment, the logic of that statement:

We burn witches.
We burn wood.
    Therefore, witches burn because they are made of wood.

Wood floats.
A duck floats.
    Therefore, to float, wood and a duck must weigh the same.
     Therefore, if a woman weighs the same as a duck, she must be made of wood.
      Therefore, she is a witch.

One of the things that is cool about this sketch is that, despite the flaws in the reasoning (Including, but not limited to: 1. all wood things burn, but not all things that burn are made of wood, so a flammable witch need not be wooden!, and 2. objects that float don't need to weigh the same; they merely need to be less dense than water -- so be wary of "very small rocks. . ."), there is a logic, albeit somewhat skewed, to this reasoning.  This is a cautionary tale, as faulty reasoning can sometimes be introduced into court proceedings, as a means of swaying the jury.  The antidote for much of this is physical evidence.

One line, "She turned me into a newt," followed by the qualifier, "I got better," calls the whole idea of eyewitness testimony into question.  In the past, however, eyewitness testimony would often have been enough.

From the first day, however,

(Click the image above to see the video again.)

we saw that perhaps eyewitness testimony was not always the most reliable form of evidence.  Once again, to quote Locard's comment about physical evidence,

This is evidence that does not forget. It is not confused by the excitement of the moment. It is not absent because human witnesses are. It is factual evidence. Physical evidence cannot be wrong, it cannot perjure itself, it cannot be wholly absent.

As such, loss of memory, confusion due to "the excitement of the moment," are real issues.  Eyewitness testimony can, indeed, be wrong, either due to human error, or due to deliberate perjury.  Eyewitness testimony may be "wholly absent," but even if it does exist, eyewitness testimony, just like a confession, is no longer enough in a court of law.

In the late 19th century, up through the modern day, physical evidence has become the cornerstone of most criminal proceedings.  It is important to understand that there are many types of evidence used in modern forensics.  I say "modern" because changes in technology have greatly expanded the types of evidence available to the crime scene investigator.  To borrow, once again, from Locard:

"Not only his fingerprints or his footprints, but his hair, the fibers from his clothes, the glass he breaks, the tool mark he leaves, the paint he scratches, the blood or semen he deposits or collects." microscopes

This course will require us to use a microscope, as many details are invisible to the naked eye.
For a sense of scale, see Cell Size & Scale:  http://learn.genetics.utah.edu/content/begin/cells/scale/
(Just drag the bar right to increase, and left to decrease, the magnification.
Note, also, the scale that appears in the upper left hand corner!).

Imagine a world without microscopes, and much of this evidence is not useful.  Even the nature of an obvious form of evidence, blood, has changed dramatically over the course of the last 150 years.  From Sherlock Holmes we know that old blood stains used to be indistinguishable from other stains:

"Are they blood stains, or rust stains, or fruit stains, or what are they?"

TYPES OF EVIDENCE:

Purposes of Physical Evidence

    Identification - to link a person to a crime

    Comparison - to find if multiple pieces of evidence have a common origin:

• Ballistics to determine if they were from the same gun

• Fingerprints of residents of a house need to be taken to determine which fingerprints are foreign

• Individual characteristics (e.g., ridge marks on fingerprints) are crucial here

• Class characteristics (e.g., race and gender)

    Probability is used to determine the frequency of an event

All of this eventually leads to crime scene reconstruction

Reliable methods of inquiry possess characteristics of:
(From Forensic Science, 2nd Edition, p. 8)

• Integrity

• Competence

• Defensible Technique (Note the connection to the courtroom here!)

• Relevant Experience

Expert Witnesses are used, but they can only be called expert by a Judge.

Scientific Method (From Forensic Science, 2nd Edition, p. 8)

• Help distinguish evidence from coincidence without ambiguity.

• Allow alternative results to be ranked by some principle basic to the sciences applied.

• Allow for certainty considerations wherever appropriate through this ranking of relevant available alternatives.

• Disallow hypotheses more extraordinary than the facts themselves.

• Pursue general impressions to the level of specific details.

• Pursue testing by breaking hypotheses (alternative explanations) into their smallest logical components, risking one part at a time.

• Allow tests either to prove or disprove alternative explanations (Hypotheses)