Expectation of k-th order statistic of Negative Binomial Distribution












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Let $ X_1, X_2,...$ be i.i.d $NB(k,q)$. I am interested in calculating the expectation of their k-th order statistic $X_{k:n}$. From my understanding of order statistics, the CDF of $X_{k:n}$ is given as
$$ P[X_{k:n} leq x] = sum_{j=k}^n {n choose j} (F(x))^j(1-F(x))^{n-j}$$



From CDF, the PMF can be derived as:
$$P[X_{k:n} = x] = P[X_{k:n} leq x] - P[X_{k:n} leq x-1] $$



and the expectation :
$$E[X_{k:n}] = sum_{x = k}^infty x P[X_{k:n} = x]$$



I fear the direct calculation in such manner would involve complex algebra. Is there any simple way to derive such expectation. I also read the "The Order Statistics of the Negative Binomial Distribution" by D. H. Young (link to the paper - The Order Statistics of the Negative Binomial Distribution). Although, the author uses the recurrence relation to calculate m-th moments.



Any help in this matter, even a simple algebra trick would be much appreciated.










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    $begingroup$


    Let $ X_1, X_2,...$ be i.i.d $NB(k,q)$. I am interested in calculating the expectation of their k-th order statistic $X_{k:n}$. From my understanding of order statistics, the CDF of $X_{k:n}$ is given as
    $$ P[X_{k:n} leq x] = sum_{j=k}^n {n choose j} (F(x))^j(1-F(x))^{n-j}$$



    From CDF, the PMF can be derived as:
    $$P[X_{k:n} = x] = P[X_{k:n} leq x] - P[X_{k:n} leq x-1] $$



    and the expectation :
    $$E[X_{k:n}] = sum_{x = k}^infty x P[X_{k:n} = x]$$



    I fear the direct calculation in such manner would involve complex algebra. Is there any simple way to derive such expectation. I also read the "The Order Statistics of the Negative Binomial Distribution" by D. H. Young (link to the paper - The Order Statistics of the Negative Binomial Distribution). Although, the author uses the recurrence relation to calculate m-th moments.



    Any help in this matter, even a simple algebra trick would be much appreciated.










    share|cite|improve this question









    $endgroup$















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      0


      1



      $begingroup$


      Let $ X_1, X_2,...$ be i.i.d $NB(k,q)$. I am interested in calculating the expectation of their k-th order statistic $X_{k:n}$. From my understanding of order statistics, the CDF of $X_{k:n}$ is given as
      $$ P[X_{k:n} leq x] = sum_{j=k}^n {n choose j} (F(x))^j(1-F(x))^{n-j}$$



      From CDF, the PMF can be derived as:
      $$P[X_{k:n} = x] = P[X_{k:n} leq x] - P[X_{k:n} leq x-1] $$



      and the expectation :
      $$E[X_{k:n}] = sum_{x = k}^infty x P[X_{k:n} = x]$$



      I fear the direct calculation in such manner would involve complex algebra. Is there any simple way to derive such expectation. I also read the "The Order Statistics of the Negative Binomial Distribution" by D. H. Young (link to the paper - The Order Statistics of the Negative Binomial Distribution). Although, the author uses the recurrence relation to calculate m-th moments.



      Any help in this matter, even a simple algebra trick would be much appreciated.










      share|cite|improve this question









      $endgroup$




      Let $ X_1, X_2,...$ be i.i.d $NB(k,q)$. I am interested in calculating the expectation of their k-th order statistic $X_{k:n}$. From my understanding of order statistics, the CDF of $X_{k:n}$ is given as
      $$ P[X_{k:n} leq x] = sum_{j=k}^n {n choose j} (F(x))^j(1-F(x))^{n-j}$$



      From CDF, the PMF can be derived as:
      $$P[X_{k:n} = x] = P[X_{k:n} leq x] - P[X_{k:n} leq x-1] $$



      and the expectation :
      $$E[X_{k:n}] = sum_{x = k}^infty x P[X_{k:n} = x]$$



      I fear the direct calculation in such manner would involve complex algebra. Is there any simple way to derive such expectation. I also read the "The Order Statistics of the Negative Binomial Distribution" by D. H. Young (link to the paper - The Order Statistics of the Negative Binomial Distribution). Although, the author uses the recurrence relation to calculate m-th moments.



      Any help in this matter, even a simple algebra trick would be much appreciated.







      order-statistics negative-binomial






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      asked Dec 3 '18 at 0:21









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