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June 21st, 2018, 07:02 PM   #11
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Quote:
 Originally Posted by Devans99 - the murderer had blood on his clothes. Makes him 70% likely to be the killer - the murderers prints are on the knife. Makes him 90% likely to be the killer
Change the order:
- the murderers prints are on the knife. Makes him ?% likely to be the killer
- the murderer had blood on his clothes. Makes him ?% likely to be the killer

Last edited by skipjack; June 21st, 2018 at 08:40 PM.

 June 21st, 2018, 07:12 PM #12 Senior Member   Joined: Jun 2018 From: UK Posts: 103 Thanks: 1 The order does not matter the answer comes out the same: 90% + 10% x 70% = 97% chance guilty
 June 21st, 2018, 09:02 PM #13 Global Moderator   Joined: Dec 2006 Posts: 20,285 Thanks: 1967 Some of your terms are undefined. Also, you initially used the past tense, but then switched to using present tense for no reason. Can you make a simpler example based on rolling a die, say, and give the justification for each step?
 June 22nd, 2018, 02:58 AM #14 Senior Member   Joined: Jun 2018 From: UK Posts: 103 Thanks: 1 It’s hard to give a die example. To state the general method I’m using: Proposition A is x% likely because of fact 1 Proposition A is y% likely because of fact 2 How likely is proposition A taking into account fact 1 and fact 2? x% + (100% - x%) x y% So a simple example: Proposition A - ‘the dog is happy’. Fact 1 - A dog psychologist tells you 60% dogs are happy Fact 2 - The dog is wagging his tail (80% dogs happy if wagging tail) 60% + 40% x 80% = 92%
 June 22nd, 2018, 10:21 AM #15 Math Team   Joined: Oct 2011 From: Ottawa Ontario, Canada Posts: 13,958 Thanks: 991 There are 100 dogs. 60 (60%) are happy. If a dog wags tail: 80% chance it's happy. To be happy, must be a dog from the 60 that are happy. 60*.80 = 48 So if Rin-Tin-Tin wags tail, there's a 48% chance he's happy.
June 22nd, 2018, 10:37 AM   #16
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Quote:
 Originally Posted by Denis There are 100 dogs. 60 (60%) are happy. If a dog wags tail: 80% chance it's happy. To be happy, must be a dog from the 60 that are happy. 60*.80 = 48 So if Rin-Tin-Tin wags tail, there's a 48% chance he's happy.
You have answered a slightly different question to what I asked: the dog does not have to be HAPPY AND WAGGING its tail, I just asked if the dog is happy.

The answer must be at least 80% because the dog is wagging his tail...

 June 22nd, 2018, 10:57 AM #17 Math Team   Joined: Oct 2011 From: Ottawa Ontario, Canada Posts: 13,958 Thanks: 991 See ya later buddy....this is kinda too confusing!! I don't have a damn dog anyway
 June 22nd, 2018, 12:34 PM #18 Senior Member   Joined: Jun 2018 From: UK Posts: 103 Thanks: 1 OK I found the error I’m making. Thanks for all the help!
 June 22nd, 2018, 01:32 PM #19 Math Team   Joined: Oct 2011 From: Ottawa Ontario, Canada Posts: 13,958 Thanks: 991 Please share it with us; may help another student...
 June 24th, 2018, 02:14 PM #20 Senior Member   Joined: Jun 2018 From: UK Posts: 103 Thanks: 1 Revised calculation below. Proposition 1 - There was a creator god Big Bang is evidence for creator at 60% Fine tuning is evidence for the creator 75% probability so: 60% + 40% x 75% = 90% Prime mover is evidence for the creator 25% probability so: 90% + 10% x 25% = 92.5% chance of a creator god is To double check, I’ve done the inverse proposition below: Proposition 2 - there was not a creator Big Bang is evidence against no creator 40% Fine tuning is evidence against no creator 25% probability so: 40% x 25% = 10% Prime mover is evidence against no creator 75% probability so: 10% x 75% = 7.5% chance of no creator god Last edited by skipjack; June 24th, 2018 at 02:54 PM.

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