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Statistics Probability - How do I do this?

  1. Sep 20, 2008 #1
    Bonus Question on Extra Credit work...

    How can I do this, where do I start? I haven't taken stats before so I'm just curious to see if someone could show me how:

    A study considered risk factors for HIV infections among IV drug users. It found that 40% of users who had less than or equal to 100 injections per month (light users) and 55% of users who had greater than 100 injections per month (heavy users) were HIV positive.

    Suppose we have a group of 10 light users and 10 heavy users.

    What is the probability that exactly 3 of the 20 users are HIV positive?

    I'm interested in the methodology and the solution if possible.

    Thanks for your help!!
  2. jcsd
  3. Sep 22, 2008 #2


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    Consider the number of choice of 3 +ve's from the groups. (0,3),(1,2),(2,1),(3,0) are the four choices possible respectively from light and heavy groups. Find the probability of each choice and add. For groups, number of +ve's follow Bin(10,0.4) and Bin(10,0.55) respectively.
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