Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Estimators math problem

  1. Oct 13, 2008 #1
    1. The problem statement, all variables and given/known data

    A library has been given 3 books. these books carry volume numbers X1, X2 and X3 where
    X1<X2<X3. But it is not known how many volumes there are altogether in the set. Suppose there are n volumes, numbered 1,2,3,...n in the set, and the 3 volumes in the library are regarded as a random sample from this total of n. Two estimators of n are proposed:

    Y=2(3X3-X1)
    Z=2X2-1
    Consider the case where n is 3. show that the value of Y must be 4 and that the value of Z must be 3.

    2. Relevant equations



    3. The attempt at a solution

    I know i have to find X1 and X2 and X3 but i dont know how to start.

    Thank you!!
     
  2. jcsd
  3. Oct 13, 2008 #2

    D H

    User Avatar
    Staff Emeritus
    Science Advisor

    Re: Estimators

    There is only one possible set of volume numbers for the case n=3. What is it?
     
  4. Oct 14, 2008 #3
    Re: Estimators

    Yes, that is 1,2 and 3

    but how does help to find X1, X2 and X3
    (sorry if this is a stupid question)
    Thank you.
     
  5. Oct 14, 2008 #4

    D H

    User Avatar
    Staff Emeritus
    Science Advisor

    Re: Estimators

    You are picking an ordered set of three elements from an ordered set of n elements. For example, with n=4 there are three possible selections: (X1,X2,X3) = (1,2,3), (1,2,4), or (2,3,4). With n=5, the number of combinations jumps to 6, 10 for n=6.

    There is only one possibility with n=3, so there is only one possibility for the estimators.
     
  6. Oct 14, 2008 #5

    HallsofIvy

    User Avatar
    Science Advisor

    Re: Estimators

    Reread the problem and THINK. The library has three volumes of a set. The set only HAS 3 volumes! What are they numbered?

    However, Y= 2(3X3-X1) is NOT 4. Did you mean Y= (3X3-X1)/2?
     
  7. Oct 14, 2008 #6
    Re: Estimators

    Why isnt (1,3,4) a possible selection??
     
  8. Oct 14, 2008 #7

    HallsofIvy

    User Avatar
    Science Advisor

    Re: Estimators

    Have you ever seen a 3 book set numbered "volume 1", "volume 3", "volume 4"? Don't you think that would cause people to wonder where "volume 2" was?
     
  9. Oct 14, 2008 #8
    Re: Estimators

    Yeah but how come there could be such thing as (1,2,4) ...where's vol 3?? :)

    I'm sorry i dont think im getting the point of how to get X1, X2 and X3.
     
  10. Oct 14, 2008 #9

    D H

    User Avatar
    Staff Emeritus
    Science Advisor

    Re: Estimators

    There is no volume 4. Reread the problem statement that you supplied. Think.
     
  11. Oct 14, 2008 #10

    Gokul43201

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Re: Estimators

    Let's do this one volume at a time.

    X1 is the smallest volume number (since you are told X1<X2<X3). What must it be?

    PS: Remember we are dealing with the case whee n = 3, i.e., the series has only 3 volumes.
     
    Last edited: Oct 14, 2008
  12. Oct 14, 2008 #11

    HallsofIvy

    User Avatar
    Science Advisor

    Re: Estimators

    Who said there could be "such a thing as (1, 2, 4)"? If there are only 3 volumes in the set, then they are numbered volume 1, volume 2, volume 3!
     
  13. Oct 15, 2008 #12
    Re: Estimators

    Oh ok ok ok, so there's only 3 volumes X1 X2 and X3 and n=3 so this means X1=1
    X2=2 and X3=3

    is that right?
     
  14. Oct 16, 2008 #13

    Gokul43201

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Re: Estimators

    Yes.
     
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook