Thread: Bayes Help DESPERATELY Needed!!!

Please Please HELP!!! This is based around Bayes and I've spent so much time trying to figure this out.


A crime occurs within an isolated community. A bloodstain is left at the crime scene. Within the adult male population, 71% is White, 16% are Afro-Caribbean and the remainder are Asian.

The offender is male according to a witness and no other information was given.
The bloodstain has genotype ABC12.
Probabilities of someone from the ethnic groups within the relevant population having this genotype are: White, p=0.025; Afro-Caribbean, p= 0.023 and Asian, p=0.012.

A suspect has been arrested: a white male with matching DNA evidence to the scene of the crime.

1) What are the prior odds that an White male committed the crime?

2) Using the information and DNA evidence, what is the Likelihood Ratio in this case?

3) Assume no match between the crime sample and the suspect’s sample. What would the Likelihood Ratio be?

Please help me (with workings too) as soon as possible, a prize for the winner and become a hero of mine for life!!!

Originally Posted by mathyou

Please Please HELP!!! This is based around Bayes and I've spent so much time trying to figure this out.


A crime occurs within an isolated community. A bloodstain is left at the crime scene. Within the adult male population, 71% is White, 16% are Afro-Caribbean and the remainder are Asian.

The offender is male according to a witness and no other information was given.
The bloodstain has genotype ABC12.
Probabilities of someone from the ethnic groups within the relevant population having this genotype are: White, p=0.025; Afro-Caribbean, p= 0.023 and Asian, p=0.012.

A suspect has been arrested: a white male with matching DNA evidence to the scene of the crime.

1) What are the prior odds that an White male committed the crime?

2) Using the information and DNA evidence, what is the Likelihood Ratio in this case?

3) Assume no match between the crime sample and the suspect’s sample. What would the Likelihood Ratio be?

Please help me (with workings too) as soon as possible, a prize for the winner and become a hero of mine for life!!!

1) Definition: Prior odds = ratio of the prior probabilities. So won't it be (0.71)(0.29) .....?

2) Likelihood ratio = { Pr(matching DNA | white male) }/{Pr(matching DNA | not a white male) }.

3) Likelihood ratio = { Pr(non-matching DNA | white male) }/{Pr(non-matching DNA | not a white male) }.

Originally Posted by mr fantastic

1) Definition: Prior odds = ratio of the prior probabilities. So won't it be (0.71)(0.29) .....?

2) Likelihood ratio = { Pr(matching DNA | white male) }/{Pr(matching DNA | not a white male) }.

3) Likelihood ratio = { Pr(non-matching DNA | white male) }/{Pr(non-matching DNA | not a white male) }.

Can you do it with the figures in the places and the answers that should be there because I'm so confused. I'm going to cry I think because I'm so desperate.

Thanks.

M.

Originally Posted by mathyou

Can you do it with the figures in the places and the answers that should be there because I'm so confused. I'm going to cry I think because I'm so desperate.

Thanks.

M.

But just like in part (1), you're given the numbers. eg. Pr(matching DNA | white male) = 0.025 etc. I'm not sure where your confusion is. Have you studied conditional probability in class?