B

#### Balkys

##### Guest

*Monotone likelihood ratio for logistic distribution $f(x;\theta) = e^{-x - \theta} (1 + e^{-x - \theta})^{-2}$*

This is a question from Problems 9.4 of

*An Introduction to Probability and Statistics*by Rohatgi.

Let $X$ have logistic distribution with the PDF $$f(x;\theta) = e^{-x - \theta} (1 + e^{-x - \theta})^{-2}, x \in \mathbb{R}.$$ Does ${f}$ have a monotone likelihood ratio?

I have tried to look at this related question, but the theory was beyond what I have learned.

**My attempt:**Let $\theta_1, \theta_0 \in \mathbb{R}$. Let $\theta_1 > \theta_0.$

Consider the likelihood ratio, $T_{NP}(\mathbf{x}) = \dfrac{f_n(\mathbf{x};\theta_1)}{f_n(\mathbf{x};\theta_0)} = \dfrac{ e^{-n \bar{x} - n \theta_1} \prod (1+e^{-x_i - \theta_1})^{-2} }{ e^{ -n \bar{x} - n \theta_0} \prod (1+e^{-x_i - \theta_0})^{-2} }.$

After simplifying more, I get $T_{NP}(\mathbf{x}) = e^{n(\theta_0 - \theta_1)} \prod \left(\dfrac{1+e^{-x_i - \theta_0}}{1+e^{-x_i - \theta_1}}\right)^2.$

I found that $\dfrac{d}{dx} \dfrac{1+e^{-x - \theta_0}}{1+e^{-x - \theta_1}}$ was negative for all $x$.

I tried to relate this to what I had done so far. I used that $\log$ is monotone increasing. Taking the log of the likelihood ratio, I get that the log-likelihood ratio is decreasing in $\sum \log \left(\dfrac{1+e^{-x_i - \theta_0}}{1+e^{-x_i - \theta_1}}\right).$ However, I am not sure how to get the required test statistic that is independent of the parameter $\theta$.

I think that a relevant result is that for a distribution from the exponential family, $f(x;\theta) = c(\theta)h(x)\exp(\pi(\theta)T(x))$, the likelihood ratio is monotone in $T(x)$.

I tried to use this by writing $f(x;\theta) = \exp{(-x-\theta)} \exp{(-2\log(1+e^{-x-\theta}))}.$ But I cannot identify $T(x)$ from this form.

Could someone please help me? Thank you very much.

SolveForum.com may not be responsible for the answers or solutions given to any question asked by the users. All Answers or responses are user generated answers and we do not have proof of its validity or correctness. Please vote for the answer that helped you in order to help others find out which is the most helpful answer. Questions labeled as solved may be solved or may not be solved depending on the type of question and the date posted for some posts may be scheduled to be deleted periodically. Do not hesitate to share your thoughts here to help others.