# Bayesian Aproach: Infering the N and $\theta$ values from a binomial distribution

I am doing a homework about infering the N value of a binomial distribution for my Bayesian Statistics Course and I have seen a paper in Biometrika magazine published in 1988 for doing so. The question is how to simulate the N and $\theta$ values based on their respective marginal posteriors. The properties about the model are explained below:

$$X\sim Binom(N,\theta)$$ The priori distribution of N:$$N\sim Poisson(\mu)$$ $$\lambda = \mu\theta$$(The logic here is that it may be easier to formulate a prior considering the unconditional expectation of the observations, rather than the mean of the unobserved N).
The $\lambda$ and $\theta$ a prior are distributed as:$$\lambda \sim Gamma(\kappa_{1},\kappa_{2})$$ $$\theta\sim Beta(\alpha,\beta)$$

After doing the procedings found on the paper which this link is below, the N marginal posterior is the following one: $p(N|x_{1},x_{2},..,x_{n})\propto(N!)^{-1}\Gamma(N+\kappa_1)\{\prod_{i=1}^5\dbinom{N}{x_i}\} \int_0^1 \! \frac{\Gamma(\alpha+\beta)}{\Gamma(\alpha)\Gamma(\beta)}\theta^{-N+S+\alpha-1}(1-\theta)^{nN-S+\beta-1}(\theta^{-1}+\kappa_{2})^{(N+\kappa_{1})} \, \mathrm{d}\theta$.

How can create a random variable generator from that marginal probability function?

Then, I suppose that this integral:$\int_0^1 \! \frac{\Gamma(\alpha+\beta)}{\Gamma(\alpha)\Gamma(\beta)}\theta^{-N+S+\alpha-1}(1-\theta)^{nN-S+\beta-1}(\theta^{-1}+\kappa_{2})^{(N+\kappa_{1})} \, \mathrm{d}\theta$ represents part of the marginal posterior of $\theta$ but I don´t know if this is true, My second question is that: Which could be the marginal distribution function for $\theta$? and How can I create a random variable function for getting $\theta$ values?.

I attached the paper I 've working with, the only thing I added is a Beta prior distributión for $\theta$.

http://ftp.stat.washington.edu/raftery/Research/PDF/bka1988.pdf

• Your post doesn't contain a clearly defined question. Nov 25, 2013 at 4:36
• I agree with Glen. Please, give a better description of the problem. Like $X\mid N=n,\Theta=\theta\sim\mathrm{Bin}(n,\theta)$. Which priors have you used for $N$ and $\Theta$?
– Zen
Nov 25, 2013 at 4:42
• Thank you for taking care of my issue. $N \sim Poisson(\mu)$ where $\mu$ is unknown, so $\lambda = \mu\theta$ (The logic here is that it may be easier to formulate a prior considering the unconditional expectation of the observations, rather than the mean of the unobserved N). $\theta ~ Beta\sim(\alpha,\beta)$. The model is hierarchical because $\lambda ~Gamma(\kappa_{1},\kappa_{2})$. The thing is that I don´t know how to obtaina random variable generator function using the inverse of this marginal posterior distribution. I don´t know either if the integral contained there would be Nov 25, 2013 at 4:58
• a marginal posterior for $\theta$ Nov 25, 2013 at 5:02
– Sycorax
Nov 25, 2013 at 15:09

The hierarchical model seems to be $$\begin{eqnarray} X_i\mid N=n,\Theta=\theta,\Lambda=\lambda &\sim& \mathrm{Bin}(n,\theta) \qquad\qquad\qquad i=1,\dots,m \\ N\mid\Theta=\theta,\Lambda=\lambda &\sim& \mathrm{Poisson}(\lambda/\theta) \\ \Theta &\sim& \mathrm{Beta}(\alpha,\beta) \\ \Lambda &\sim& \mathrm{Gamma}(\kappa_1,\kappa_2) \, , \\ \end{eqnarray}$$ in which $\alpha,\beta,\kappa_1$, and $\kappa_2$ are known. Defining $X=(X_1,\dots,X_m)$ and $x=(x_1,\dots,x_m)$, and using Bayes' Theorem, we have $$\begin{eqnarray} f_{N,\Theta,\Lambda\mid X}(n,\theta,\lambda\mid x) &\propto& f_{X\mid N,\Theta,\Lambda}(x\mid n,\theta,\lambda)\; f_{N,\Theta,\Lambda}(n,\theta,\lambda) \\ &=& \left(\prod_{i=1}^m f_{X_i\mid N,\Theta,\Lambda}(x_i\mid n,\theta,\lambda)\right) \; f_{N\mid\Theta,\Lambda}(n\mid\theta,\lambda) \; f_\Theta(\theta) \; f_{\Lambda}(\lambda) \, .\\ \end{eqnarray}$$ Therefore, $$\begin{eqnarray} f_{N\mid X}(n\mid x) &=& \int_0^1 \int_0^\infty f_{N,\Theta,\Lambda\mid X}(n,\theta,\lambda\mid x)\,d\lambda\,d\theta \\ &\propto& \frac{1}{n!} \left( \prod_{i=1}^m {n\choose x_i}\right) \int_0^1 \int_0^\infty \theta^{t-n+\alpha -1} (1-\theta)^{mn-t+\beta-1} \\ && \qquad\qquad\qquad\qquad\qquad\times\lambda^{n+\kappa_1-1} \exp\left(-\left(\frac{1+\kappa_2\theta}{\theta}\right)\lambda\right)\,d\lambda\,d\theta \, , \end{eqnarray}$$ in which $t=\sum_{i=1}^m x_i$. The integration in $\lambda$ is easy. Just identify the kernel of a $$\mathrm{Gamma}\left( n+\kappa_1, \frac{1+\kappa_2\theta}{\theta} \right)$$ distribution, and adjust the normalization constant. Doing this, we have $$f_{N\mid X}(n\mid x) \propto \frac{\Gamma(n+\kappa_1)}{n!} \left( \prod_{i=1}^m {n\choose x_i}\right) \int_0^1 \frac{\theta^{t+\alpha-mn-1}(1-\theta)^{mn-t+\beta-1}}{(1+\kappa_2\theta)^{n+\kappa_1}} \,d\theta \, .$$ Of course, the last expression holds for $n\geq \max\{x_1,\dots,x_m\}$. If this is not clear, write the indicators $I_{\{0,\dots,n\}}(x_i)$ in the binomials explicitly.

If we use an improper prior $f_\Lambda(\lambda)\propto 1/\lambda$, the integrations are easy. Using an uniform prior $(\alpha=\beta=1)$ for $\Theta$, we find the closed form $$f_{N\mid X}(n\mid x) \propto \frac{n\,\Gamma(t+1)\Gamma(mn-t+1)}{\Gamma(mn+2)} .$$

How do we normalize this? What is the Bayes estimate with quadratic loss $\mathrm{E}[N\mid X=x]$? Can we find analytically at least the posterior mode? Can we sample from this distribution to find an estimate and a credible set?

I didn't check the algebra. Please, do it.

• THANK YOU FOR THAT Zen, It can can not be described better. The thing is how can create values of N based on the marginal posterior distribution stated above? and how can I create values of $\theta$ based on its marginal posterior distribution? Nov 25, 2013 at 16:23
• One approach, which eschews integration entirely, would just compute the bivariate posterior density across a grid of points $(\theta, N)$ and then numerically sum over $N$ or $\theta$ to get the desired marginal distribution.
– Sycorax
Nov 25, 2013 at 16:29
• Use the results in the answer to compute the full conditionals $N\mid\Theta,\Lambda,X$, and $\Theta\mid N,\Lambda,X$, and $\Lambda\mid N,\Theta,X$. If you can sample from these full conditionals, code a Gibbs sampler. Discarding the simulated $\lambda$'s you will have a sample from $N,\Theta\mid X$, and so on.
– Zen
Nov 25, 2013 at 16:55
• I think the original right question would have been, how can I obtain full conditionals $N\mid\Theta,\Lambda$ , $\Theta\mid N,\Lambda,X$, $\Lambda\mid N,\Theta,X$? , I feel that the integral $\int_0^1 \frac{\theta^{t+\alpha+\kappa_1-1}(1-\theta)^{mn-t+\beta-1}}{(1+\kappa_2\theta)^{n+\kappa_1}} \,d\theta \,$ is very difficult to compute. Moreover, Is this integral useful to OBTAIN the full conditional $\Theta\mid N,\Lambda,X$? Nov 25, 2013 at 18:09
• Check with your teacher if you can use an improper prior for $\Lambda$. I've just added that to the answer.
– Zen
Nov 25, 2013 at 18:15