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You're both wrong. This is a Bayesian problem. If "Man" is the event that a man transferred, and "Data" is the observation of a man drawn after the event from the class, then: Pr(Man | Data) = Pr(Data | Man) * Pr( Man) / Pr(Data) Where: Pr(Data|Man) = 8/11 Pr(Man) = 5/9, the prior probability of event Pr(Data) = Pr(Data|Man)*Pr(Man) + Pr(Data|Woman)*Pr(Woman) So the answer for (a) is (5/9*8/11)/(5/9*8/11 + 4/9*7/11) = 5*8/(5*8+4*7) ~= 0.5882353 The answer for (b) is of course 1-(a) __________________________________________ Mike Weale, PhD Senior Lecturer in Statistical Genetics King's College London, Dept. of Medical& Molecular Genetics 8th Floor, Tower Wing, Guy's Hospital, London SE1 9RT Tel: +44 (0)20 7188 2601 Fax: +44 (0)20 7188 2585 Web: www.kcl.ac.uk/medicine/research/divisions/gmm/sections/clusters/bse/weale SGU info: www.kcl.ac.uk/medicine/research/divisions/gmm/sections/clusters/statisticalgenetics.aspx SEGEG info: www.kcl.ac.uk/medicine/research/divisions/gmm/sections/clusters/bse/weale/segeg On 17/08/2011 17:24, Cartwright, Michael wrote: > The questions are not asking about the student who is randomly selected from the intermediate course, but is asking merely about the student who was transferred. The number in the intermediate course is therefore irrelevant and the answers are 5/9 and 4/9 respectively. > > > >> -----Original Message----- >> From: A UK-based worldwide e-mail broadcast system mailing >> list [mailto:[log in to unmask]] On Behalf Of Rodrigo Briceņo >> Sent: 17 August 2011 15:53 >> To: [log in to unmask] >> Subject: I need an advice >> >> Dear allstat listers. I am discussing with a friend the >> proper way to solve a probabilities exercise. For me is very >> simple, but his solution made me think if I'm doing something >> wrong. I will write the exercise and my proposed answer. If >> anyone on the list believe that the solution is incorrect, >> please tell me why. >> >> Exercise: >> In a basic English course there are 5 men and 4 women. In the >> intermediate English course there are 7 men and 3 women. >> Finally in the advanced English course there are 4 men and 4 >> women. An student is randomly selected from the basic course >> and is transferred to the intermediate course. If an student >> is randomly selected from the intermediate course and the >> student is a man: >> >> a. What is the probability that the student transferred was a man? >> b. What is the probability that the student transferred was a woman? >> >> My answer: >> >> a. 5/9 * 8/11 >> b. 4/9 * 7/11 >> >> Thanks for your cooperation. >> >> -- >> Rodrigo Briceņo >> Economist >> [log in to unmask] >> MSN: [log in to unmask] >> SKYPE: rbriceno1087 >> >> You may leave the list at any time by sending the command >> >> SIGNOFF allstat >> >> to [log in to unmask], leaving the subject line blank. >> > You may leave the list at any time by sending the command > > SIGNOFF allstat > > to [log in to unmask], leaving the subject line blank. > You may leave the list at any time by sending the command SIGNOFF allstat to [log in to unmask], leaving the subject line blank.