How Well Would You Do On A Jury?

5 Questions | By Newscientist | Last updated: May 18, 2018

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If you were on a jury for a murder trial, would you make the right decision? Trials rely on statistical evidence, which sometimes be confusing. This quiz tests how well you understand the stats that are used. Return to article

Questions and Answers

1.

A man has been stabbed to death at his home in a city, and a suspect is on trial for the crime.The murder weapon has been recovered, and a sample of DNA has been obtained from the handle.The suspect's DNA profile matches that on the handle. An expert witness says that the probability of anyone else being a DNA match is 1 in 3 million.What are the chances that the DNA on the knife came from someone other than the suspect?

Discuss
- A.
  
  1 in 3 million
- B.
  
  More than 1 in 3 million
- C.
  
  Less than 1 in 3 million
- D.
  
  We don't have enough information to say
2.

A man has been murdered. Multiple witnesses saw that the murder was committed by a white man with black hair and a black woman with dyed yellow hair, though none was close enough to see their faces and identify them.An expert calculates, based on census data, that the odds of finding another interracial couple matching the description are 1 in 12 million.The suspects are a married couple; the man is white with black hair, and the woman is black with dyed yellow hair. They live in the correct area and have no alibi.What are the chances that the couple are innocent of the murder?

Discuss
- A.
  
  Less than 1 in 12 million
- B.
  
  1 in 12 million
- C.
  
  More than 1 in 12 million
- D.
  
  We don't have enough information to say
3.

A man has been murdered, and various pieces of evidence mean that we can be certain that the murderer had a particular disease.The disease is rare; only 1 in 10,000 people have it.The suspect has been tested for the disease, using a test that is 99 per cent accurate, and the test was positive.What is the probability that the suspect really has the disease?

Discuss
- A.
  
  1 in 1: certain
- B.
  
  99 in 100
- C.
  
  1 in 100
- D.
  
  1 in 101
- E.
  
  Less than 1 in 101
4.

A woman has been murdered, and her husband is on trial for the crime.Some years before, the husband had been convicted of domestic violence towards his wife. However, an expert witness states that of all the women who are abused by their boyfriends/husbands, fewer than 1 in 1000 of them end up being murdered by them.What do these facts tell us about the husband; is he more or less likely to be guilty?

Discuss
- A.
  
  The fact that the husband abused the wife means, other things being equal, that he is more likely to be the murderer
- B.
  
  The fact that the husband abused the wife means, other things being equal, that he is less likely to be the murderer
- C.
  
  The fact that the husband abused the wife makes no difference one way or the other
- D.
  
  The fact that the husband abused the wife may tell us something about whether he is guilty, but the facts as presented are not enough for us to work out what it tells us.
5.

Two children have died in the same family. Their parents are on trial, accused of murdering them.The defence claims that the children both died of Sudden Infant Death Syndrome, or "cot death".An expert witness testifies that the odds of one child dying of cot death, in a family like the one on trial, is 1 in 8500.Hence, he argues, the probability that both died of cot death is that probability multiplied by itself: 1 in 73 million.What is the probability that the two children did both die of cot death?

Discuss
- A.
  
  Less than 1 in 73 million
- B.
  
  1 in 73 million
- C.
  
  Between 1 in 73 million and 1 in 8500
- D.
  
  1 in 8500
- E.
  
  More than 1 in 8500

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