8/11/2019 First Paper 2014

1/2

First Paper Fall, 2014

9/11/2014

The setting is a jury trial for murder. The victim died as the result of a

fatal gun shot. The defendant is charged with a capital crime. During thetrial, the following forensic evidence is introduced.

The fatal bullet was removed from the victim. A chemical analysis ofthe metals in the bullet was undertaken. The same kind of analysis wasundertaken for a bullet from a box of ammunition known to have been in thepossession of the defendant. The following information about the forensicprocedures used was provided.

1. The probability of a match when the bullets are the same .98.

2. The probability that the bullets are the same when there is a match is

.82

3. The probability that the bullets are the same and there is a match is.49

At the crime scene, scrapings from under the victims fingernails containedsome skin fragments. DNA from those skin fragments was compared to thedefendants DNA. The following information about the forensic proceduresused was provided.

1. The probability of a match when the two samples of DNA are from thesame person is .99.

2. The probability that the two samples of DNA are from the same personwhen there is a match is .93

3. The probability that the person is the same and there is a match is .74.

1

8/11/2019 First Paper 2014

2/2

Questions

1. For the bullet analysis, which probability or probabilities is (are) di-rectly relevant for the case? Why?

2. For the DNA analysis, which probability or probabilities is (are) di-rectly relevant for the case? Why?

3. Assume that the two forensic procedures (i.e., the bullet tests and DNAtests) are independent. Is this a plausible assumption here? Why orwhy not? What do you mean by independence?

4. The prosecutor claims that the forensics strongly support both the

claim that the bullet fired in the homicide came from the box of am-munition known to be in the possession of the defendant and that thetwo DNA samples (i.e., from the crime scene and from the defendant)came from the same person. If the prosecutor is not committing any ofthe prosecution fallacies we discussed, what is the relevant probability,assuming that the two forensic procedures are independent?

5. From the facts about the crime given to you above, what would beanother important piece of non-forensic evidence easily obtained thatcould well be decisive. (e.g., Dont make up that the defendant con-fessed or that there was an eye witness.)

6. Using the facts about the crime and your answers so far, what doyou conclude about the guilt or innocence of the defendant? If youwere a juror, how would you vote? Explain your reasoning drawing ondescriptive validity and statistical conclusion validity.

For all of your probability calculations, show your work. If you writeclearly, you can do a good job in well under two pages of single spaced text.

2