Forensic science the basics chapter 3
Unknown evidence:
Evidence whose source or ownership is not known at the time it is discovered.
False negative test:
A chemical test that turns out negative even though it should have been positive.
False positive test:
A chemical test that turns out positive even though it should have been negative.
Class evidence
Evidence that cannot be associated with one particular object or person. It can only be put into a group of similar pieces of evidence
Physical evidence:
Evidence that consists of objects or people or materials taken from them.
Demonstrative evidence
Evidence that does not arise from the crime itself but is created to illustrate or explain evidence. Crime scene drawings or photographs are examples
Relevant:
Evidence that is both material and probative that pertains to the case at hand and tends to prove or disprove something about the case.
Real evidence:
Evidence that is generated directly from criminal activity.
Known evidence:
Evidence whose source or ownership is known at the time it is collected.
Individual-Class Evidence
fingerprints and fibers, are examples of individual and class evidence, respectively.
There are two major types of analysis
identification and comparison. The first, identification, is always performed on all evidence, known or unknown. Identification is a process of discovering chemical, biological, and/or physical properties of a piece of evidence.
Positive control:
A material or chemical that would be expected to respond positively to a particular chemical test.
Negative control:
A material, usually a matrix, that would be expected to respond negatively to a particular chemical test.
Probative:
A piece of evidence that tends to prove or disprove a fact or assertion.
Evidence:
Anything that will make a issue more or less true than it would be without the evidence.
Class Evidence
Class Evidence Why Not Individual? Tiny glass or paint fragments Too small to fracture match, no unique chemical or physical properties Soils No classification system. Too much variability between nearby samples Hairs and fibers No unique characteristics. Hair can be individualized if DNA present illicit drugs, explosives, fire residues, etc. Can identify all chemical components but no unique Charactaristic
Material:
Evidence is material if it pertains to the particular case that is being tried or investigated.
Nonphysical evidence:
Evidence such as eye witness or expert testimony or interpretation of an analytical test.
Individual evidence:
Evidence that can be associated with one particular person or object.
Individualizable Evidence
Individualizable Evidence Individualizing Characteristics Fingerprints Level 2 and 3 minutiae details Handwriting Individual, unique style and characteristics Shoe prints and tire treads Unique characteristics that develop with time, damage and wear Large pieces of paint or glass or paper, etc. Fracture or tear match with unique edge characteristics DNA Rarity of any DNA type makes probability of two people with same DNA extremely improbable
Physical-Nonphysical Evidence
Physical evidence consists of objects or things. Nonphysical evidence is verbal testimony about a crime or it may be someone's actions during a crime
Positive control
Positive controls are used to make sure that the chemicals in a test are functioning properly so that there are no false negative results. Negative controls are used to make sure that only the target of the test will react with the reagents in the test and that a false positive reaction is not being seen.
Real-Demonstrative Evidence
Real evidence is that which is generated by criminal activity. It is found at the crime scene or elsewhere and pertains to the crime.Demonstrative evidence, on the other hand, is created to help explain or clarify real evidence. It is produced after the crime and not by the crime.
false positives example
The phenolphthalein test is run by moistening a sterile swab and applying it to the stained area until transfer of some of the stain is noted on the swab. The swab is then tested with the phenolphthalein reagent. If the reagent turns purple, it is possible that blood is present. In reality, this test could come out positive for two reasons. The first, of course, is that the stain is really blood. But it could be that the stain is not blood but that there is something in the fibers of the shirt that causes the test to give a false positive reading. A false positive test is one that comes out positive when it should be negative. How do we detect which is occurring in this case? The best way is to take some fibers from the shirt that have no blood on them and test them exactly the same way as with the fibers that contain the stain. The fibers without the stain should come out negative when the test is run on them. If that is the case, then the positive result on the fibers with the stain must be due to the stain, which we would then presume to be blood. We call the fibers that have no stain on them; negative controls
Identification of evidence
The process of describing and documenting chemical, biological, and/or physical characteristics of evidence.
DNA Typing Situation
The science of DNA analysis has developed to the point that it is usually considered to be individual evidence
Known-Unknown Evidence
The technical term for this source information is provenance. The value of every piece of real evidence arises from its association with someone or something that was involved in the crime. Crime scene reconstruction depends upon making these associations. We use the term unknown evidence to refer to evidence that is discovered at a crime scene and its origin or source is not known
negative control
a known substance or material that would be expected to yield a negative result to a particular test A negative control should always be run whenever a chemical test is being run on a substance that is mixed with a matrix such as the shirt. This guards against the matrix being the reason for the positive test
Classification of Evidence
• Physical-nonphysical • Real-demonstrative • Known-unknown • Individual-class
