# Inverse gamma convergence in probability

I am trying to prove the consistency of MLE for a beta distribution. The problem now reduces to the following:

Assume $Y=\frac n X$ and $X$ ~ Gamma(n,$\frac 1 \theta$), prove that $Y$ converges to $\theta$ in probability when $n$ goes to infinity.

I set up the proof by the definition of convergence in probability but cannot prove it.

• What definition did you use? What prevents you from carrying out the proof?
– whuber
Jan 31 '16 at 18:23
• The limit of $\rm{P}(|Y-\theta|<\epsilon)$ is 1 for any $\epsilon > 0$ when $n$ goes to infinity. Basically, this probability can be calculated as the difference of CDF of a gamma distribution, but it is hard for me to find a closed form of that CDF when integrate from $\frac n {\theta + \epsilon}$ to $\frac n {\theta - \epsilon}$. Jan 31 '16 at 18:25
• You might want to try to approximate that probability with a normal probability, which in the limit is exact of course. Jan 31 '16 at 18:42
• @JohnK Thanks for your suggestion, I know that Gamma can be approximated by normal when $n$ is large. But I am not comfortable with this approximation because this is not the classical approach of proof for convergence in prob. Jan 31 '16 at 18:51
• Chebyshev's inequality does not require that the constant is the mean. You can prove that $$E\left(Y - c \right)^2 \geq a^2 P\left[ | Y - c | \geq a \right]$$ for any constants $a>0$ and $c$. Jan 31 '16 at 19:17

This is also a direct consequence of the Law of Large numbers: if $X_n\sim\mathcal{G}(n,\theta)$, $X$ can be written as $$X_n=Y_1+\cdots+Y_n\qquad Y_i\stackrel{\text{iid}}{\sim}\mathcal{E}(\theta)$$ Therefore, $$\dfrac{X_n}{n}=\dfrac{\sum_{i=1}^n Y_i}{n}\stackrel{\text{a.s.}}{\longrightarrow}\mathbb{E}[Y_1]=\dfrac{1}{\theta}$$ and thus $$\dfrac{n}{X_n}\stackrel{\text{a.s.}}{\longrightarrow}\theta$$
The answer is using Chebyshev's Inequality to prove that: $$\rm{P} (|Y-\theta| \geq \epsilon) \leq \frac 1 {\epsilon^2} E[(Y-\theta)^2]$$ Since Y is equal to $\frac n X$ where $X$ is a Gamma distributed random variable ($X$ ~ Gamma ($n$,$\frac 1 \theta$)), one can calculate that: $$E[(Y-\theta)^2]=\frac {n^2\theta^2} {(n-1)(n-2)} - \frac {2n\theta^2} {n-1} + \theta^2$$ which goes to 0 when $n$ goes to infinity. This demonstrates that $$\rm{P} (|Y-\theta| \geq \epsilon)=0$$ when $n$ goes to infinity. Therefore, Y is a consistent estimator of $\theta$ (Y converge to $\theta$ in probability).