Results 1 to 6 of 6
Like Tree1Thanks
  • 1 Post By biffboy

Math Help - Need help with discrete probabilities

  1. #1
    Member
    Joined
    Sep 2009
    Posts
    104
    Thanks
    1

    Need help with discrete probabilities

    Hey guys, I need help with this question:

    Suppose the random variable x = 1, 2, 3, 4, 5, 6. In addition suppose:

    p(1)=p(2) and p(3)=p(4)=p(5)=p(6)

    If the mean μ=4.00, then P(μ - 2σ ≤ x ≤ μ + 2σ) is:
    The correct answer is 1.000, but I'm having trouble figuring out how to do it

    from the question, i can derive two formulas

    (1) 2a + 4b = 1
    (2) 3a + 18b = 4.00

    Formula (2) was derived from the expected value rule

    not sure how to continue on from here... any help?
    Follow Math Help Forum on Facebook and Google+

  2. #2
    Senior Member
    Joined
    Mar 2012
    From
    Sheffield England
    Posts
    440
    Thanks
    76

    Re: Need help with discrete probabilities

    Solve the simultaneous equations. You now know the probability of each value. Work out the standard deviation from the formula
    (SD)^2=sum of (px^2)-(mean)^2.
    Follow Math Help Forum on Facebook and Google+

  3. #3
    Member
    Joined
    Sep 2009
    Posts
    104
    Thanks
    1

    Re: Need help with discrete probabilities

    What do you mean by this:

    Quote Originally Posted by biffboy View Post
    (SD)^2=sum of (px^2)-(mean)^2.
    σ2 = [Σ(x-)2]/n

    what is (px^2) supposed to be?
    Follow Math Help Forum on Facebook and Google+

  4. #4
    Senior Member
    Joined
    Mar 2012
    From
    Sheffield England
    Posts
    440
    Thanks
    76

    Re: Need help with discrete probabilities

    Let p1 = prob. x=1, p2= prob.x=2 and so on. We need the sum of px^2
    That is p1(1)^2+p2(2)^2..........p6(6)^2 Then (sd)^2 =(sum of px^2)-(mean)^2. Perhaps you haven't seen this formula before but it is the correct one to use here.
    Follow Math Help Forum on Facebook and Google+

  5. #5
    Member
    Joined
    Sep 2009
    Posts
    104
    Thanks
    1

    Re: Need help with discrete probabilities

    0.0833*12 + 0.0833*22 + 0.20833*32 + 0.20833*42 + 0.20833*52 + 0.20833*62 = 18.333

    is this the expected value formula you're using?
    Follow Math Help Forum on Facebook and Google+

  6. #6
    Senior Member
    Joined
    Mar 2012
    From
    Sheffield England
    Posts
    440
    Thanks
    76

    Re: Need help with discrete probabilities

    Yes that's correct. So for the standard deviation subtract (mean)^2 (which is 16) and take square root. I get 1.528
    In probability distributions of this kind learn the formulae
    Mean = Sigma px and (SD)^2= sigma px^2-(mean)^2
    Thanks from snypeshow
    Follow Math Help Forum on Facebook and Google+

Similar Math Help Forum Discussions

  1. Marginal Probabilities of a discrete 2 dimensional variable
    Posted in the Advanced Statistics Forum
    Replies: 1
    Last Post: November 23rd 2011, 04:12 PM
  2. Replies: 3
    Last Post: September 22nd 2011, 10:46 AM
  3. Approximate joint probabilities using marginal probabilities
    Posted in the Advanced Statistics Forum
    Replies: 0
    Last Post: June 23rd 2011, 05:04 AM
  4. Probabilities
    Posted in the Discrete Math Forum
    Replies: 1
    Last Post: December 14th 2009, 12:20 PM
  5. Probabilities
    Posted in the Statistics Forum
    Replies: 1
    Last Post: October 14th 2009, 09:31 AM

Search Tags


/mathhelpforum @mathhelpforum