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Homework Help: Stats question: Item collection

  1. Sep 24, 2012 #1
    1. The problem statement, all variables and given/known data

    Suppose that I'm collecting cards, and that in a complete collection there are m items.
    When buying a new card, there's an equal probability that the card is any of those m cards.

    Let X be the number of cards I need to buy in order to get a complete collection

    What is the Expectation/Ex of X? What is the Standard Deviation?

    2. Relevant equations

    Let [itex]X = \sum^{m}_{i=1} X_{i} [/itex], where [itex] X_{i} [/itex] is the number of cards I need to buy in order to get a new type of card when I already have i - 1 different types of cards

    3. The attempt at a solution

    I figure this problem would involve probability mass function, but to be honest I'm stuck as I haven't had any probability or stats in over 10 years.

    Any good pointers on how to go on with this problem?
  2. jcsd
  3. Sep 24, 2012 #2


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    haven't worked it, but say you have n>m cards, then what is the probability of having m different cards might be a place to start...
  4. Sep 24, 2012 #3
    After doing some reading, this looks like something that falls under Geometric Distribution. Correct?
  5. Sep 24, 2012 #4

    Ray Vickson

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    Google the Coupon Collector's Problem.

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