1. Not finding help here? Sign up for a free 30min tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Conditional Probability Statistics

  1. Jan 14, 2013 #1
    1. Assume that there is a 50% chance of hard drive damage if a power line to which a computer is connected is hit during an electrical storm. There is a 5% chance that an electrical storm will occur on any given summer day in a given area. If there is a 0.1% chance that the line will be hit during a storm, what is the probability that the line will be hit and there will be hard drive damage during the next electrical storm in this area?



    2. Bayes Theorem? P(A|B) = (P(B|A) * P(A))/ P(B)



    3. I'm thinking it has something to do with Bayes Theorem...
    P(hard drive damage) = 0.5
    P(electrical storm) = 0.05
    P(line hit) = 0.001

    I started off by figuring out what is the probability of the line getting hit:
    P(l|es) = (0.001 * 0.05)/ 0.05 = 0.001
    but I'm not sure what I should do next and if my P(l|es) is correct.
     
  2. jcsd
  3. Jan 14, 2013 #2

    haruspex

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member
    2016 Award

    The circumstance is 'the next storm', and you know the probability of a hit during such a storm. The probability of a storm is irrelevant.
    So you have: prob of a hit; prob of damage given a hit.
     
  4. Jan 14, 2013 #3

    HallsofIvy

    User Avatar
    Staff Emeritus
    Science Advisor

    Here is how I would do this problem:
    Since the problem says "during the next electrical storm", the "There is a 5% chance that an electrical storm will occur on any given summer day in a given area" is irrelevant- we are given that there is a storm. Suppose there are 1000000 electrical storms. There is a .1% (.001) chance of a line being hit by lightning. So out of 1000000, how many lines are hit by lightning? Of those, 50%= .5 will result in hard drives damaged. So of those 1000000 storms, how many drives are damaged? What proportion is that?


     
  5. Jan 19, 2013 #4
    Thank You!!!
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: Conditional Probability Statistics
Loading...