View unanswered posts | View active topics It is currently 24 May 2018

Reply to topic  [ 4 posts ] 
 Probability problem using Bayes formula 
Author Message

Joined: 27 Feb 2013
Posts: 8
This is taken from Examwise Vol 2 for the 2011 L1 cert. SS02. Q1.
The probability of passing an eye test for people over 70 years old is 80%, and failing is 20%. Fr people under 70 years old the rates are 95% and 5% respectively. 15% of the people tested are over 70 years old.
A report is found of someone failing the test. What is the probability that they are over 70 years old?
Can someone work this out for me? The reason being is I think there is an error in the answer given in the book, and I want to check it.

17 Mar 2013

Joined: 27 Feb 2013
Posts: 12
andytrader Wrote:
First you need to find probability of failing
using conditional:

Prob(Fail) = Prob(Fail|=70) + Prob(Fail|
= 20%(15%) + 5%(85%) = 7.25%

Then use Bayes:

Prob(=70|Fail) = [ Prob(Fail|=70)/Prob(Fail) ] *
Prob (=70)

= [20%/7.25%]*15%

= 41.4%
I thought we could get the same answer using tree diagram. If I try using tree diagram for this, I don’t get the same answer. Am I missing anything? Could someone demonstrate the same problem using tree diagram?

17 Mar 2013

Joined: 20 Apr 2016
Posts: 1
i solved it and had 44.4%,what is the answer provided in whatever source of this question :)

20 Apr 2016

Joined: 22 Aug 2016
Posts: 3

22 Aug 2016
Display posts from previous:  Sort by  
Reply to topic   [ 4 posts ] 

Who is online

Registered users: Bing [Bot], Google [Bot], Majestic-12 [Bot]

You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum
You cannot post attachments in this forum

Search for:
Jump to:  
Powered by phpBB® Forum Software © phpBB Group

Copyright © 2004-2017 All Rights Reserved