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drcoby · Sep 24, 2010, 07:52 AM

A truth serum has the property that 90% of the guilty suspects are properly judged while of course 10% guilty suspects are improperly. On the other hand, innocent suspects are misjudged 1% of the time. A suspect was elected from a group of suspects of which only 5% committed a crime and the serum indicates he is guilty. What is the propbability that is he is innocent?

Unknown008 · Sep 24, 2010, 08:53 AM

Draw a probability tree and all the answers should appear easily. Post what you get. :)

drcoby · Sep 24, 2010, 09:02 AM

I got P=0.1743

is that correct

Unknown008 · Sep 24, 2010, 09:23 AM

Great! Well done! :)

drcoby · Sep 24, 2010, 09:36 AM

OK I've got another question:

A certain federal agency employs three consulting firms( A,B,C) with probabilities 0.40, 0.35 and 0.25 respectively. From past experience, it is known that the prob of cost overruns for the firms are 0.05, 0.03 and 0.15 respectively. Suppose a cost overrun is experienced by the agency.
1/ What is the prob that the consulting firms involved is C?
2. What is the prob that is company A?

I got it wrong I need to use bayes theorem for it
But I don't understand could you please explain it to me
Thanks

Unknown008 · Sep 24, 2010, 09:45 AM

I don't know the Bayes' Theorem, sorry... :(

But that's how I'd do:

$P(C\ |\ overrun) = \frac{P(C\ \cap\ overrun)}{P(overrun)}$

P(overrun) = (0.4 x 0.05) + (0.35 x 0.03) + (0.25 x 0.15) = 0.068

So, the answer for (a) would be = $\frac{0.25 \times 0.15}{0.068} = 0.55147... \approx 0.551$

Then, for (b), it'll be: $\frac{0.4 \times 0.05}{0.068} = 0.29411... \approx 0.294$

EDIT: forgot a math tab... fix'd now

drcoby · Sep 24, 2010, 10:05 AM

Hey monoir to mari bon serye manz ar li..

Unknown008 · Sep 24, 2010, 10:08 AM

Lol, didn't know you were Mauritian too =P

Where do you live? =P

drcoby · Sep 24, 2010, 10:12 AM

Canada to coner cav probabilite mone fer sa secondaire la mo concordia university mo bizin faire ca mari bez ca.. Lol mo bizin reapprane ca encore

drcoby · Sep 24, 2010, 10:13 AM

Canada to coner caV probabilite mone fer sa secondaire la mo concordia university mo bizin faire ca mari bez ca.. Lol mo bizin reapprane ca encore ine blier imper.. mais bon sey manz r li la..

Unknown008 · Sep 24, 2010, 10:21 AM

Lol, OK, mo dan upper 6 la. ~19 zours avan lexamen >.<

Si to ena problem encore, pas hesiter pose questions :)

Me montrer ki to finn seyer selman. Nou pas siposer donne response koum sa mem. Ok? :)

drcoby · Sep 24, 2010, 10:23 AM

Ah OK corek... sinon ki luniversite to pense pour aller ek en koi
Moi mo dans engineering

Unknown008 · Sep 24, 2010, 10:27 AM

=/ pas encor koner... medecine pou possibleman trop cher... franchement, mo pas koner :(

drcoby · Sep 24, 2010, 10:28 AM

Fer to best to cav gagne la bourse.. bonne chance..

Unknown008 · Sep 24, 2010, 10:31 AM

Thanks :)

Bon courage a toi aussi :)

drcoby · Sep 24, 2010, 07:24 PM

Merci!