## Monday, 27 April 2015

### Parametric Inference: Likelihood Ratio Test by Example

Hypothesis testing have been extensively used on different discipline of science. And in this post, I will attempt on discussing the basic theory behind this, the Likelihood Ratio Test (LRT) defined below from Casella and Berger (2001), see reference 1.
Definition. The likelihood ratio test statistic for testing $H_0:\theta\in\Theta_0$ versus $H_1:\theta\in\Theta_0^c$ is $$\label{eq:lrt} \lambda(\mathbf{x})=\frac{\displaystyle\sup_{\theta\in\Theta_0}L(\theta|\mathbf{x})}{\displaystyle\sup_{\theta\in\Theta}L(\theta|\mathbf{x})}.$$ A likelihood ratio test (LRT) is any test that has a rejection region of the form $\{\mathbf{x}:\lambda(\mathbf{x})\leq c\}$, where $c$ is any number satisfying $0\leq c \leq 1$.
The numerator of equation (\ref{eq:lrt}) gives us the supremum probability of the parameter, $\theta$, over the restricted domain (null hypothesis, $\Theta_0$) of the parameter space $\Theta$, that maximizes the joint probability of the sample, $\mathbf{x}$. While the denominator of the LRT gives us the supremum probability of the parameter, $\theta$, over the unrestricted domain, $\Theta$, that maximizes the joint probability of the sample, $\mathbf{x}$. Therefore, if the value of $\lambda(\mathbf{x})$ is small such that $\lambda(\mathbf{x})\leq c$, for some $c\in [0, 1]$, then the true value of the parameter that is plausible in explaining the sample is likely to be in the alternative hypothesis, $\Theta_0^c$.

Example 1. Let $X_1,X_2,\cdots,X_n\overset{r.s.}{\sim}f(x|\theta)=\frac{1}{\theta}\exp\left[-\frac{x}{\theta}\right],x>0,\theta>0$. From this sample, consider testing $H_0:\theta = \theta_0$ vs $H_1:\theta<\theta_0$.

## Saturday, 25 April 2015

### SAS®: Getting Started with PROC IML

Another powerful procedure of SAS, my favorite one, that I would like to share is the PROC IML (Interactive Matrix Language). This procedure treats all objects as a matrix, and is very useful for doing scientific computations involving vectors and matrices. To get started, we are going to demonstrate and discuss the following:
• Creating and Shaping Matrices;
• Matrix Query;
• Subscripts;
• Descriptive Statistics;
• Set Operations;
• Probability Functions and Subroutine;
• Linear Algebra;
Above outline is based on the IML tip sheet (see Reference 1). So to begin on the first bullet, consider the following code:

## Thursday, 16 April 2015

### Python and R: Basic Sampling Problem

In this post, I would like to share a simple problem about sampling analysis. And I will demonstrate how to solve this using Python and R. The first two problems are originally from Sampling: Design and Analysis book by Sharon Lohr.

### Problems

1. Let $N=6$ and $n=3$. For purposes of studying sampling distributions, assume that all population values are known.

 $y_1 = 98$ $y_2 = 102$ $y_3=154$ $y_4 = 133$ $y_5 = 190$ $y_6=175$

We are interested in $\bar{y}_U$, the population mean. Consider eight possible samples chosen.

 Sample No. Sample, $\mathcal{S}$ $P(\mathcal{S})$ 1 $\{1,3,5\}$ $1/8$ 2 $\{1,3,6\}$ $1/8$ 3 $\{1,4,5\}$ $1/8$ 4 $\{1,4,6\}$ $1/8$ 5 $\{2,3,5\}$ $1/8$ 6 $\{2,3,6\}$ $1/8$ 7 $\{2,4,5\}$ $1/8$ 8 $\{2,4,6\}$ $1/8$