Investigation of Crime Scene Evidence Test 1

a DNA analyst, a detective, a fingerprint examiner, a prosecutor

Who is not a trier of fact in the list below?

-member of the jury

-judge

-a detective

-a fingerprint examiner

-a prosecutor

-a DNA analyst

questioned items typically come from a crime scene, while known items have a chain of custody that link them to a specific source or person.

What is the difference between questioned items and known items?

• questioned items typically come from a crime scene, while known items have a chain of custody that link them to a specific source or person.

• questioned items are those that are returned by an AFIS system or similar computer system for comparison with a known item from a crime scene. 

• two objects may appear identical at the level at which we observe them or can remember them (e.g. two new pennies produced by the same mint in the same year).

• although each item is unique, so is every impression left by that object, and sometimes the impressions are degraded enough to confuse the impression with the wrong item.

If every item is unique, why is it sometimes difficult to make the case for an identification?

• two objects may appear identical at the level at which we observe them or can remember them (e.g. two new pennies produced by the same mint in the same year).

• although each item is unique, so is every impression left by that object, and sometimes the impressions are degraded enough to confuse the impression with the wrong item.

Type 1 error

an error that typically occurs when you accuse someone via a test result bu they are not actually guilty

Type 2 error

a test result that exonorates someone who was actually guilty

positive control

a substance that is used to verify that a test reagent is functioning properly by producing a result

negative control

a substance that is used to verify that a test reagent is functioning properly by producing no result

false positive

a result where you declare that a substance is present but it is, no present

false negative

a situation where your test produces no result but in fact the tested-for substance was actually present

latent print

the kind of print left unintentionally, typically by a suspect or victim at a crime scene

exemplar print

a high quality (usually) record or impression taken on a scanner or using ink

we multiply the prior odds by the lieklihood ratio

For example, based on other evidence, you might consider the prior odds that the child was abused as 2:1, meaning you think that the child was twice as likely to be abused as not abused.  

How do you create the posterior odds?

• we multiply the prior odds by the likelihood ratio

• we add the prior odds to the likelihood ratio

• we divide the prior odds by the likelihood ratio

a blood pattern analyst concluses that a pattern is forward spatter froma gunshot because they observe brain matter next to the sample

Which of the following is NOT examples of base rate fallacies???

• a fingerprint examiner receive a comparison from a detective who almost always gets the rigth suspect.

• a TSA agent is more likely to inspect the bag from a person wearing a turban.

• a blood pattern analyst concluses that a pattern is forward spatter from a gunshot because they observe brain matter next to the sample

• a medical examiner determines a cause of death by the boyfriend of a mother is assumed to be murder while a cause of death by a grandmother caretaker is an accident.

you tend to think about biases more and you end up sucumbing to them more

Why can’t experts overcome biases by knowledge and willpower?

knowing the make and model of a firearm when doing a firearms comparison

what is not an example of a reference materials bias?

• looking at an exemplar print from a suspect in custody and using it to infer the location of minutia in a noisy are of the latent print (called ‘reverse comparison’)

• knowing the make and model of a firearm when doing a firearms comparison

• a fingerprint database suggests a print for comparison that comes from a database that only contains convicted criminals

• using a suspect’s DNA profile to guide the interpretation of a complex DNA mixture

the focus of attention, what visual features the participant thinks is relevant for the task, where the eyes point

what does an eye tracker record?

Mated, non mated

What items belong to the ground truth?

method

When reading an article, which section contains the details of the study design and data collection?

No, because these studies only measure the overall accuracy of a discipline

When conducting ‘Black Box’ tests, do the study authors care about how the subjects came to their conclusion?

erroneous exclusions

There are 2 kinds of errors: erroneous identifications (false positives), and erroneous exclusions (false negatives). Which error rate was larger in the Black Box study?

the evidence is not probative, because it does not bear directly on the question of who might have committed the crime

If you run a DNA test that places a suspect at their own home and this is presented to a jury, why might that be a problem?

probative evidence

evidence that is useful in proving something in the case

exculpatory evidence

evidence that tends to show the defendant is not guilty

foundational evidence

evidence required to show that a method meets admissibility standards

the number or correct identifications would decrease and the number of erroneous identifications would decrease

suppose the examiner becomes more risk averse with their identification threshold. What would happen to the correct identifications and erroneous identifications?

expert immunity

experts are inherently objective and accurate, so their judgments don’t need to be questioned

technological protection

advanced tools or technologies make error or bias impossible

ethical issues

ethical training or professional codes are enough to prevent misconduct or bias

bias blind spot

other people are biased, but I am not

illusion of control

experts or organizations believe they can fully control or monitor the sources of error or misconduct

bad apples

errors or misconduct only happen because of a few bad individuals

we multiply the prior odds times the likelihood ratio to produce the posterior odds

How do we update our prior odds with the likelihood ratio to produce the posterior odds?

Ninhydrin

develops latent prints on porous surfaces like paper by reacting with amino acids in latent print residue. in a fume hood, the specimens are submerged in the ninhydrin solution and then air dried.

glue fuming

fumes from cyanoacrylate ester adhesives (super glue) will develop latent prints by binding the proteins in the prints. the super glue is heated in the presence of water to create fumes.

point-counting standard

how many points of comparison are required before a positive conclusion could be reached

base rate fallacy

When forensic experts or decision-makers focus too much on a particular piece of evidence, they might ignore how rare or common the event actually is in the broader context.