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I What is meant by this notation?

  1. Mar 10, 2016 #1
    when saying the probability distribution of X is f(x) = (3 x) this is to be like vector notation where 3 is above x but i can't write it like this here. what is meant by this notation ?
     
  2. jcsd
  3. Mar 10, 2016 #2

    BvU

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    There is a vecor-like notation for the number of combinations ("n over k") but I don't know if that's what you are referring to:
    $$\dbinom 3 x = {3!\over x!\, (3-x)!}$$(to me it seems a bit weird as a probability distribution...)
     
  4. Mar 10, 2016 #3
    that's right thank you !
    i don't know why do they represent probability distributions by this notation
     
  5. Mar 11, 2016 #4

    Mark44

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    This notation is commonly used for binomial distributions.

    ##\dbinom 3 x## is usually read as "3 choose x", the number of ways of choosing x items from a group of 3 of those items.
     
  6. Mar 11, 2016 #5

    Mark44

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    Yes. It's the number of combinations of 3 things taken x at a time. It's usually read as "3 choose x."
     
  7. Mar 11, 2016 #6

    FactChecker

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    One problem with that interpretation of the notation is that the distribution function will not total 1. Is it possible that the definition of f(x) is missing a multiplier?
     
  8. Mar 11, 2016 #7

    mathman

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    What is the definition of f(x)? The combination term is a coefficient of the probability term for exactly x.
     
  9. Mar 11, 2016 #8

    FactChecker

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    The original post stated: "the probability distribution of X is f(x) = (3 x) ". If we interpret that as f(x) = 3Cx, then it does not total 1.
     
  10. Mar 12, 2016 #9

    HallsofIvy

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    I suspect that the OP is completely misreading what is said and that it really is something that involves [itex]\begin{pmatrix}3 \\ x \end{pmatrix}[/itex] such as the binomial distribution with n= 3, [itex]f(x)= \begin{pmatrix}3 \\ x \end{pmatrix} p^x (1- p)^{3- x}[/itex] for x= 0, 1, 2, or 3.
     
  11. Mar 12, 2016 #10

    FactChecker

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    Good catch. That has to be it.
     
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