# What will be Minimum Variance Unbiased Estimator for this?

• Mar 23rd 2011, 11:23 PM
amul28
What will be Minimum Variance Unbiased Estimator for this?
Let X1,X2,...,Xn be a random sample from a distribution with p.d.f.,
f(x;θ)=θ2xe^(−xθ);0<x<∞,θ>0
Obtain minimum variance unbiased estimator of θ and examine whether it is attained?

MY WORK: (Nerd)

Using MLE i have found the estimator for θ=2/(x bar)

Or as

X∼Gamma(2,θ)
So
E(X)=2θ
E(X/2)=θ
so can i take x/2 as unbiased estimator of θ. I'm stuck and confused need some help.

Thank u.(Nerd)
• Mar 23rd 2011, 11:35 PM
Sambit
One way is by using Cramer-Rao Lower Bound This will also tell you whether it is attained.
• Mar 24th 2011, 12:07 AM
amul28
i got $\displaystyle CRLB=\frac{\theta^2}{2n}$ when i took $\displaystyle \psi'(\theta)=\theta$

and now if i take $\displaystyle V(X)=2\theta^2$ as X follows $\displaystyle Gamma(2,\theta)$
it does not attain!!!
Is there ant mistake.
• Mar 31st 2011, 11:17 PM
matheagle
$\displaystyle \Gamma (2)=1$ that might be your error.