How to get the value of 'scaled' binomial distribution?












0












$begingroup$


People kindly told me that there is not a equivalent popular distribution for $aX$ when $X$ is distributed as Binomial, but it is just a 'scaled' distribution. Here, $a$ is a positive constant.



Question



How to get the value of 'scaled' binomial distribution? Do I need to use $Gamma$ function? A simpler way is appreciated.



To be more precise, I want to calculate $$g(x) = frac{1}{|a|}f(frac xa) = {n choose x/a} p^{frac xa} (1-p)^{(n-frac xa)},$$ where $f(x)$ is $B(n,p)$ and $a$ is positive constant.



What is the simplest way to calculate $g(x)$?



Background



I'm considering this question under the same condition with this question, where $X$ is distributed as $Binomial$ and I want to know the distribution of $aX$. $a$ is a real and positive number.



I know
$aX sim frac{1}{|a|}f(frac xa)$ ($f$ means the distribution of $X$),so I tried to calculate the probability of $aX$ by $frac{1}{|a|}f(frac xa) = {n choose x/a} p^{frac ax} (1-p)^{(n-frac xa)}$. But, this was not easy for me because $x/a$ is not a integer.



I think with $Gamma$ function $nCr$ can be calculated for any real number $r (< n)$. But is there any straightforward way to calculate 'scaled' binomial distribution?










share|cite|improve this question











$endgroup$












  • $begingroup$
    You may want to look at the beta function—en.wikipedia.org/wiki/Beta_function—as it provides a way to extend the binomial distribution to real values. (It's indicated near the end of the "Properties" section.)
    $endgroup$
    – Brian Tung
    Apr 27 '15 at 16:54










  • $begingroup$
    So, yes, you will need to use the gamma function. I suspect there's no way around it for your application.
    $endgroup$
    – Brian Tung
    Apr 27 '15 at 16:55










  • $begingroup$
    @BrianTung thank you. This is what I'm looking for. If you replace your comment to an answer, I would give a check mark :)
    $endgroup$
    – rkjt50r983
    Apr 28 '15 at 2:15












  • $begingroup$
    Thanks for your kind offer! But I'd probably only move it to an answer if you're looking to give out a checkmark. (I don't remember if you get any points for doing that.) Otherwise, if my contribution is merely a link to another page, I generally leave it in a comment. If it helped you out, that's good enough for me.
    $endgroup$
    – Brian Tung
    Apr 28 '15 at 16:23










  • $begingroup$
    @rkjt50r983, your scale function is not a density function because it does not normalise to 1. I think you consider that as a critical issue as well.
    $endgroup$
    – Chamberlain Foncha
    Nov 12 '17 at 15:01
















0












$begingroup$


People kindly told me that there is not a equivalent popular distribution for $aX$ when $X$ is distributed as Binomial, but it is just a 'scaled' distribution. Here, $a$ is a positive constant.



Question



How to get the value of 'scaled' binomial distribution? Do I need to use $Gamma$ function? A simpler way is appreciated.



To be more precise, I want to calculate $$g(x) = frac{1}{|a|}f(frac xa) = {n choose x/a} p^{frac xa} (1-p)^{(n-frac xa)},$$ where $f(x)$ is $B(n,p)$ and $a$ is positive constant.



What is the simplest way to calculate $g(x)$?



Background



I'm considering this question under the same condition with this question, where $X$ is distributed as $Binomial$ and I want to know the distribution of $aX$. $a$ is a real and positive number.



I know
$aX sim frac{1}{|a|}f(frac xa)$ ($f$ means the distribution of $X$),so I tried to calculate the probability of $aX$ by $frac{1}{|a|}f(frac xa) = {n choose x/a} p^{frac ax} (1-p)^{(n-frac xa)}$. But, this was not easy for me because $x/a$ is not a integer.



I think with $Gamma$ function $nCr$ can be calculated for any real number $r (< n)$. But is there any straightforward way to calculate 'scaled' binomial distribution?










share|cite|improve this question











$endgroup$












  • $begingroup$
    You may want to look at the beta function—en.wikipedia.org/wiki/Beta_function—as it provides a way to extend the binomial distribution to real values. (It's indicated near the end of the "Properties" section.)
    $endgroup$
    – Brian Tung
    Apr 27 '15 at 16:54










  • $begingroup$
    So, yes, you will need to use the gamma function. I suspect there's no way around it for your application.
    $endgroup$
    – Brian Tung
    Apr 27 '15 at 16:55










  • $begingroup$
    @BrianTung thank you. This is what I'm looking for. If you replace your comment to an answer, I would give a check mark :)
    $endgroup$
    – rkjt50r983
    Apr 28 '15 at 2:15












  • $begingroup$
    Thanks for your kind offer! But I'd probably only move it to an answer if you're looking to give out a checkmark. (I don't remember if you get any points for doing that.) Otherwise, if my contribution is merely a link to another page, I generally leave it in a comment. If it helped you out, that's good enough for me.
    $endgroup$
    – Brian Tung
    Apr 28 '15 at 16:23










  • $begingroup$
    @rkjt50r983, your scale function is not a density function because it does not normalise to 1. I think you consider that as a critical issue as well.
    $endgroup$
    – Chamberlain Foncha
    Nov 12 '17 at 15:01














0












0








0





$begingroup$


People kindly told me that there is not a equivalent popular distribution for $aX$ when $X$ is distributed as Binomial, but it is just a 'scaled' distribution. Here, $a$ is a positive constant.



Question



How to get the value of 'scaled' binomial distribution? Do I need to use $Gamma$ function? A simpler way is appreciated.



To be more precise, I want to calculate $$g(x) = frac{1}{|a|}f(frac xa) = {n choose x/a} p^{frac xa} (1-p)^{(n-frac xa)},$$ where $f(x)$ is $B(n,p)$ and $a$ is positive constant.



What is the simplest way to calculate $g(x)$?



Background



I'm considering this question under the same condition with this question, where $X$ is distributed as $Binomial$ and I want to know the distribution of $aX$. $a$ is a real and positive number.



I know
$aX sim frac{1}{|a|}f(frac xa)$ ($f$ means the distribution of $X$),so I tried to calculate the probability of $aX$ by $frac{1}{|a|}f(frac xa) = {n choose x/a} p^{frac ax} (1-p)^{(n-frac xa)}$. But, this was not easy for me because $x/a$ is not a integer.



I think with $Gamma$ function $nCr$ can be calculated for any real number $r (< n)$. But is there any straightforward way to calculate 'scaled' binomial distribution?










share|cite|improve this question











$endgroup$




People kindly told me that there is not a equivalent popular distribution for $aX$ when $X$ is distributed as Binomial, but it is just a 'scaled' distribution. Here, $a$ is a positive constant.



Question



How to get the value of 'scaled' binomial distribution? Do I need to use $Gamma$ function? A simpler way is appreciated.



To be more precise, I want to calculate $$g(x) = frac{1}{|a|}f(frac xa) = {n choose x/a} p^{frac xa} (1-p)^{(n-frac xa)},$$ where $f(x)$ is $B(n,p)$ and $a$ is positive constant.



What is the simplest way to calculate $g(x)$?



Background



I'm considering this question under the same condition with this question, where $X$ is distributed as $Binomial$ and I want to know the distribution of $aX$. $a$ is a real and positive number.



I know
$aX sim frac{1}{|a|}f(frac xa)$ ($f$ means the distribution of $X$),so I tried to calculate the probability of $aX$ by $frac{1}{|a|}f(frac xa) = {n choose x/a} p^{frac ax} (1-p)^{(n-frac xa)}$. But, this was not easy for me because $x/a$ is not a integer.



I think with $Gamma$ function $nCr$ can be calculated for any real number $r (< n)$. But is there any straightforward way to calculate 'scaled' binomial distribution?







probability probability-distributions






share|cite|improve this question















share|cite|improve this question













share|cite|improve this question




share|cite|improve this question








edited Apr 13 '17 at 12:19









Community

1




1










asked Apr 27 '15 at 10:11









rkjt50r983rkjt50r983

259210




259210












  • $begingroup$
    You may want to look at the beta function—en.wikipedia.org/wiki/Beta_function—as it provides a way to extend the binomial distribution to real values. (It's indicated near the end of the "Properties" section.)
    $endgroup$
    – Brian Tung
    Apr 27 '15 at 16:54










  • $begingroup$
    So, yes, you will need to use the gamma function. I suspect there's no way around it for your application.
    $endgroup$
    – Brian Tung
    Apr 27 '15 at 16:55










  • $begingroup$
    @BrianTung thank you. This is what I'm looking for. If you replace your comment to an answer, I would give a check mark :)
    $endgroup$
    – rkjt50r983
    Apr 28 '15 at 2:15












  • $begingroup$
    Thanks for your kind offer! But I'd probably only move it to an answer if you're looking to give out a checkmark. (I don't remember if you get any points for doing that.) Otherwise, if my contribution is merely a link to another page, I generally leave it in a comment. If it helped you out, that's good enough for me.
    $endgroup$
    – Brian Tung
    Apr 28 '15 at 16:23










  • $begingroup$
    @rkjt50r983, your scale function is not a density function because it does not normalise to 1. I think you consider that as a critical issue as well.
    $endgroup$
    – Chamberlain Foncha
    Nov 12 '17 at 15:01


















  • $begingroup$
    You may want to look at the beta function—en.wikipedia.org/wiki/Beta_function—as it provides a way to extend the binomial distribution to real values. (It's indicated near the end of the "Properties" section.)
    $endgroup$
    – Brian Tung
    Apr 27 '15 at 16:54










  • $begingroup$
    So, yes, you will need to use the gamma function. I suspect there's no way around it for your application.
    $endgroup$
    – Brian Tung
    Apr 27 '15 at 16:55










  • $begingroup$
    @BrianTung thank you. This is what I'm looking for. If you replace your comment to an answer, I would give a check mark :)
    $endgroup$
    – rkjt50r983
    Apr 28 '15 at 2:15












  • $begingroup$
    Thanks for your kind offer! But I'd probably only move it to an answer if you're looking to give out a checkmark. (I don't remember if you get any points for doing that.) Otherwise, if my contribution is merely a link to another page, I generally leave it in a comment. If it helped you out, that's good enough for me.
    $endgroup$
    – Brian Tung
    Apr 28 '15 at 16:23










  • $begingroup$
    @rkjt50r983, your scale function is not a density function because it does not normalise to 1. I think you consider that as a critical issue as well.
    $endgroup$
    – Chamberlain Foncha
    Nov 12 '17 at 15:01
















$begingroup$
You may want to look at the beta function—en.wikipedia.org/wiki/Beta_function—as it provides a way to extend the binomial distribution to real values. (It's indicated near the end of the "Properties" section.)
$endgroup$
– Brian Tung
Apr 27 '15 at 16:54




$begingroup$
You may want to look at the beta function—en.wikipedia.org/wiki/Beta_function—as it provides a way to extend the binomial distribution to real values. (It's indicated near the end of the "Properties" section.)
$endgroup$
– Brian Tung
Apr 27 '15 at 16:54












$begingroup$
So, yes, you will need to use the gamma function. I suspect there's no way around it for your application.
$endgroup$
– Brian Tung
Apr 27 '15 at 16:55




$begingroup$
So, yes, you will need to use the gamma function. I suspect there's no way around it for your application.
$endgroup$
– Brian Tung
Apr 27 '15 at 16:55












$begingroup$
@BrianTung thank you. This is what I'm looking for. If you replace your comment to an answer, I would give a check mark :)
$endgroup$
– rkjt50r983
Apr 28 '15 at 2:15






$begingroup$
@BrianTung thank you. This is what I'm looking for. If you replace your comment to an answer, I would give a check mark :)
$endgroup$
– rkjt50r983
Apr 28 '15 at 2:15














$begingroup$
Thanks for your kind offer! But I'd probably only move it to an answer if you're looking to give out a checkmark. (I don't remember if you get any points for doing that.) Otherwise, if my contribution is merely a link to another page, I generally leave it in a comment. If it helped you out, that's good enough for me.
$endgroup$
– Brian Tung
Apr 28 '15 at 16:23




$begingroup$
Thanks for your kind offer! But I'd probably only move it to an answer if you're looking to give out a checkmark. (I don't remember if you get any points for doing that.) Otherwise, if my contribution is merely a link to another page, I generally leave it in a comment. If it helped you out, that's good enough for me.
$endgroup$
– Brian Tung
Apr 28 '15 at 16:23












$begingroup$
@rkjt50r983, your scale function is not a density function because it does not normalise to 1. I think you consider that as a critical issue as well.
$endgroup$
– Chamberlain Foncha
Nov 12 '17 at 15:01




$begingroup$
@rkjt50r983, your scale function is not a density function because it does not normalise to 1. I think you consider that as a critical issue as well.
$endgroup$
– Chamberlain Foncha
Nov 12 '17 at 15:01










1 Answer
1






active

oldest

votes


















0












$begingroup$

For a random variable $X$ of binomial distribution of parameters $n,p$ the possible values taken are $0,1,2, cdots, n$ with probabilities$$P(X=k)={n choose k}p^k(1-p)^{n-k}.$$
If $a$ is a real constant and $Y=aX$ then the valueas taken by $Y$ are $1a,2a,cdots , na$ with probabilities $$P(Y=ak)=P(X=k).$$
I don't know if this answers the question.



EDITED



Perhaps, the reason of confusion is that there is no pdf. The cdf is
$$F_Y(x)=P(aX<x)=Pleft(X<frac{x}{a}right)=F_Xleft(frac{x}{a}right).$$



If the derivative existed then one could get a formula similar to what you quoted for the Gaussian distribution.






share|cite|improve this answer











$endgroup$













  • $begingroup$
    thank you, but I think it is necessary to calculate ${n choose k/a}p^{k/a}(1-p)^{n-k/a}.$. More specifically, how do I define and calculate ${n choose k/a}$?
    $endgroup$
    – rkjt50r983
    Apr 27 '15 at 10:47












  • $begingroup$
    @rkjt50r938, The probabilities that $Y=ai$ are given. What else would want anybody know? The is no pdf in this case. Perhaps you want see the form of the cdf??
    $endgroup$
    – zoli
    Apr 27 '15 at 10:55










  • $begingroup$
    @rkjt50r938: I've edited my answer.
    $endgroup$
    – zoli
    Apr 27 '15 at 11:39











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1 Answer
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oldest

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1 Answer
1






active

oldest

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active

oldest

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active

oldest

votes









0












$begingroup$

For a random variable $X$ of binomial distribution of parameters $n,p$ the possible values taken are $0,1,2, cdots, n$ with probabilities$$P(X=k)={n choose k}p^k(1-p)^{n-k}.$$
If $a$ is a real constant and $Y=aX$ then the valueas taken by $Y$ are $1a,2a,cdots , na$ with probabilities $$P(Y=ak)=P(X=k).$$
I don't know if this answers the question.



EDITED



Perhaps, the reason of confusion is that there is no pdf. The cdf is
$$F_Y(x)=P(aX<x)=Pleft(X<frac{x}{a}right)=F_Xleft(frac{x}{a}right).$$



If the derivative existed then one could get a formula similar to what you quoted for the Gaussian distribution.






share|cite|improve this answer











$endgroup$













  • $begingroup$
    thank you, but I think it is necessary to calculate ${n choose k/a}p^{k/a}(1-p)^{n-k/a}.$. More specifically, how do I define and calculate ${n choose k/a}$?
    $endgroup$
    – rkjt50r983
    Apr 27 '15 at 10:47












  • $begingroup$
    @rkjt50r938, The probabilities that $Y=ai$ are given. What else would want anybody know? The is no pdf in this case. Perhaps you want see the form of the cdf??
    $endgroup$
    – zoli
    Apr 27 '15 at 10:55










  • $begingroup$
    @rkjt50r938: I've edited my answer.
    $endgroup$
    – zoli
    Apr 27 '15 at 11:39
















0












$begingroup$

For a random variable $X$ of binomial distribution of parameters $n,p$ the possible values taken are $0,1,2, cdots, n$ with probabilities$$P(X=k)={n choose k}p^k(1-p)^{n-k}.$$
If $a$ is a real constant and $Y=aX$ then the valueas taken by $Y$ are $1a,2a,cdots , na$ with probabilities $$P(Y=ak)=P(X=k).$$
I don't know if this answers the question.



EDITED



Perhaps, the reason of confusion is that there is no pdf. The cdf is
$$F_Y(x)=P(aX<x)=Pleft(X<frac{x}{a}right)=F_Xleft(frac{x}{a}right).$$



If the derivative existed then one could get a formula similar to what you quoted for the Gaussian distribution.






share|cite|improve this answer











$endgroup$













  • $begingroup$
    thank you, but I think it is necessary to calculate ${n choose k/a}p^{k/a}(1-p)^{n-k/a}.$. More specifically, how do I define and calculate ${n choose k/a}$?
    $endgroup$
    – rkjt50r983
    Apr 27 '15 at 10:47












  • $begingroup$
    @rkjt50r938, The probabilities that $Y=ai$ are given. What else would want anybody know? The is no pdf in this case. Perhaps you want see the form of the cdf??
    $endgroup$
    – zoli
    Apr 27 '15 at 10:55










  • $begingroup$
    @rkjt50r938: I've edited my answer.
    $endgroup$
    – zoli
    Apr 27 '15 at 11:39














0












0








0





$begingroup$

For a random variable $X$ of binomial distribution of parameters $n,p$ the possible values taken are $0,1,2, cdots, n$ with probabilities$$P(X=k)={n choose k}p^k(1-p)^{n-k}.$$
If $a$ is a real constant and $Y=aX$ then the valueas taken by $Y$ are $1a,2a,cdots , na$ with probabilities $$P(Y=ak)=P(X=k).$$
I don't know if this answers the question.



EDITED



Perhaps, the reason of confusion is that there is no pdf. The cdf is
$$F_Y(x)=P(aX<x)=Pleft(X<frac{x}{a}right)=F_Xleft(frac{x}{a}right).$$



If the derivative existed then one could get a formula similar to what you quoted for the Gaussian distribution.






share|cite|improve this answer











$endgroup$



For a random variable $X$ of binomial distribution of parameters $n,p$ the possible values taken are $0,1,2, cdots, n$ with probabilities$$P(X=k)={n choose k}p^k(1-p)^{n-k}.$$
If $a$ is a real constant and $Y=aX$ then the valueas taken by $Y$ are $1a,2a,cdots , na$ with probabilities $$P(Y=ak)=P(X=k).$$
I don't know if this answers the question.



EDITED



Perhaps, the reason of confusion is that there is no pdf. The cdf is
$$F_Y(x)=P(aX<x)=Pleft(X<frac{x}{a}right)=F_Xleft(frac{x}{a}right).$$



If the derivative existed then one could get a formula similar to what you quoted for the Gaussian distribution.







share|cite|improve this answer














share|cite|improve this answer



share|cite|improve this answer








edited Apr 27 '15 at 18:16

























answered Apr 27 '15 at 10:35









zolizoli

17k41945




17k41945












  • $begingroup$
    thank you, but I think it is necessary to calculate ${n choose k/a}p^{k/a}(1-p)^{n-k/a}.$. More specifically, how do I define and calculate ${n choose k/a}$?
    $endgroup$
    – rkjt50r983
    Apr 27 '15 at 10:47












  • $begingroup$
    @rkjt50r938, The probabilities that $Y=ai$ are given. What else would want anybody know? The is no pdf in this case. Perhaps you want see the form of the cdf??
    $endgroup$
    – zoli
    Apr 27 '15 at 10:55










  • $begingroup$
    @rkjt50r938: I've edited my answer.
    $endgroup$
    – zoli
    Apr 27 '15 at 11:39


















  • $begingroup$
    thank you, but I think it is necessary to calculate ${n choose k/a}p^{k/a}(1-p)^{n-k/a}.$. More specifically, how do I define and calculate ${n choose k/a}$?
    $endgroup$
    – rkjt50r983
    Apr 27 '15 at 10:47












  • $begingroup$
    @rkjt50r938, The probabilities that $Y=ai$ are given. What else would want anybody know? The is no pdf in this case. Perhaps you want see the form of the cdf??
    $endgroup$
    – zoli
    Apr 27 '15 at 10:55










  • $begingroup$
    @rkjt50r938: I've edited my answer.
    $endgroup$
    – zoli
    Apr 27 '15 at 11:39
















$begingroup$
thank you, but I think it is necessary to calculate ${n choose k/a}p^{k/a}(1-p)^{n-k/a}.$. More specifically, how do I define and calculate ${n choose k/a}$?
$endgroup$
– rkjt50r983
Apr 27 '15 at 10:47






$begingroup$
thank you, but I think it is necessary to calculate ${n choose k/a}p^{k/a}(1-p)^{n-k/a}.$. More specifically, how do I define and calculate ${n choose k/a}$?
$endgroup$
– rkjt50r983
Apr 27 '15 at 10:47














$begingroup$
@rkjt50r938, The probabilities that $Y=ai$ are given. What else would want anybody know? The is no pdf in this case. Perhaps you want see the form of the cdf??
$endgroup$
– zoli
Apr 27 '15 at 10:55




$begingroup$
@rkjt50r938, The probabilities that $Y=ai$ are given. What else would want anybody know? The is no pdf in this case. Perhaps you want see the form of the cdf??
$endgroup$
– zoli
Apr 27 '15 at 10:55












$begingroup$
@rkjt50r938: I've edited my answer.
$endgroup$
– zoli
Apr 27 '15 at 11:39




$begingroup$
@rkjt50r938: I've edited my answer.
$endgroup$
– zoli
Apr 27 '15 at 11:39


















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