I have been looking at an MCMC data augmentation question; the general form of the question is as follows:

Suppose data gathered on a process suggests $X_{i} sim text{Pois}(lambda)$ and a prior for the rate parameter is suggested as $lambda sim text{Exp}(lambda_{0})$.

The data is recorded and presented in a typical form (i.e. the number of occurrences of each value for $X_{i}$ from $0$ to $n$), however, the data gathered does not discriminate in instances where $X_{i} leq 1$ (i.e. all occurrences where $X_{i} = 0$ and $X_{i} = 1$ are grouped into one category).

Given the data, the likelihood and the prior described above, the question asks for:

The posterior form of $lambda$,

The number of occurrences where $X_{i} = 0$.

I'm not really sure how to answer this question, but I am aware that Gibbs Sampling can be used in data augmentation.

Does anybody have any information on how this could be done?

**EDIT:**

I should specify that it's primarily the second part (the number of occurrences where $X_{i} = 0$) that I'm unsure about. For the first part (the posterior form of $lambda$), given the likelihood and the prior suggested, I have reasoned (although I'm happy to be corrected):

Given:

$$ pi(lambda|vec{x}) propto p(vec{x}|lambda) times p(lambda) $$

So, for the model given above:

$$ pi(lambda|vec{x}) = frac{lambda^{sum_{i=1}^{n}x_{i}}}{sum_{i=1}^{n}x_{i}!}e^{-nlambda} times lambda_{0}e^{-lambda lambda_{0}} $$

Simplifying yields:

$$ pi(lambda|vec{x}) = frac{lambda^{sum_{i=1}^{n}x_{i}}}{sum_{i=1}^{n}x_{i}!}e^{-lambda(n+lambda_{0})}lambda_{0} $$

which is proportional to (and hence the posterior form is given by):

$$ pi(lambda|vec{x}) propto lambda^{sum_{i=1}^{n}x_{i}}e^{-lambda(n+lambda_{0})}lambda_{0} $$

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#### Best Answer

Your answer does not account for the fact that the observations equal to zero and to one are merged together: what you computed is the posterior for the complete Poisson data, $(X_1,ldots,X_n)$, rather than for the aggregated or merged data, $(X_1^*,ldots,X^*_n)$.

If we take the convention that cases when the observation $X_i^*=1$ correspond to $X_i=1$ or $X_i=0$ and the observation $X_i^*=k>1$ to $X_i=k$, the density of the observed vector $(X_1^*,ldots,X^*_n)$ is (after some algebra and factorisation) $$ pi(lambda|x_1^*,ldots,x^*_n) propto lambda^{sum_{i=1}^n x_i^*mathbb{I}(x_i^*>1)} exp{-lambda(lambda_0+n)} times {1+lambda}^{n_1} $$ where $n_1$ is the number of times the $x_i^*$'s are equal to one. The last term between brackets above is the probability to get 0 or 1 in a Poisson draw.

So this is your true/observed posterior. From there, you can implement a Gibbs sampler by

- Generating the "missing observations" given $lambda$ and the observations, namely simulating $p(x_i|lambda,x_i^*=1)$, which is given by $$ mathbb{P}(x_i=0|lambda,x_i^*=1)=1-mathbb{P}(x_i=1|lambda,x_i^*=1)=dfrac{1}{1+lambda},. $$
- Generating $lambda$ given the "completed data", which amounts to $$ lambda|x_1,ldots,x_n sim mathcal{G}(sum_i x_i + 1,n+lambda_0) $$ as you already computed.

(If you want more details, Example 9.7, p.346, in my Monte Carlo Statistical Methods book with George Casella covers exactly this setting.)

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