# Lecture 01

Course and Bayesian Analysis Introduction

Jihong Zhang

Educational Statistics and Research Methods

## Today’s Lecture Objectives

1. Introduce myself
2. Syllabus
3. Extra Course Information
4. Introduce Bayesian Analysis

but, before we begin…

## Introduce myself

Let me introduce myself first…

## Syllabus

Syllabus of ESRM 6553

## Course Time

• Monday 5PM to 7:45PM:

• 5PM to 6:15PM: First half class

• 6:15PM to 6:30PM: 15-min break

• 6:30PM to 7:45PM: Second half class

## Office Hours

• Tuesday 1:30PM to 4:30PM

• You should be able to find me in GRAD Room 109

## Materials

• I will provide R codes and slides at the weekends before next class. You may download them on Blackboard or My website (jihongzhang.org)

## Quiz: What is Bayesian?

1. How to Pronounce Bayesian (Real Life Examples!)
• “B-Asian” or “Bayes-ian”?
• What “Bayesian” mean? Assign probabilities to everything!
2. Frequentist vs Bayesian
• Example: In U.S., more Male Asian Faculty or Female Asian Faculty?
• Frequentist: If “Male:Female = 1:1 out of Asian Faculty” is fixed, how is the probability that the data happens?
• Bayesian: If I believe Male:Female = 1:2 but the data says 1:1, to what degree I need to update my mind?

## Bayesian Model Components

1. What we see: Observed Data
2. What we cannot see: Future Data, Data yet to be collected, Parameters

Take home Note I: Everything is random in Bayesian!

• Observed Data: Some random information given a unknown generation process
• Parameter: The random components controlling the generation process

Take home Note II: Every random component can be expressed as probability!

• The probability of observed data given parameters: $p(\text{Observed Data}|\text{Parameters})$ = Likelihood
• The probability of parameters: $p(\text{Parameters})$ = Prior Distribution
• The probability of parameters given observed data: $p(\text{Parameters}|\text{Observed Data})$ = Posterior Distribution*

### Bayesian Thinking Process

#### A toy example:

<<<<<<< HEAD:posts/2024-01-12-syllabus-adv-multivariate-esrm-6553/Lecture01/Lecture01.qmd I originally thought the ratio of male Asian faculty to female is about 1:2 (Prior). But the data we sampled from 2000 doctoral students suggested gender ratio is 1:1 (Data; Likelihood that the true ratio we don’t know). Based on Bayes’s rule, estimate for the ratio is probably 1:1.5 (Posterior) after combining these two statements. ======= I originally thought the ratio of male Asian faculty to female is about 1:2 (Prior). We obtained the information from 2000 doctoral candidates suggested gender ratio is 1:1 (Data; Likelihood give the true ratio we don’t know). Based on Bayes’s rule, estimate for the ratio is probably 1:1.5 (Posterior) after combining these two statements. >>>>>>> 6e75efe (update 2024 CV):posts/2024-01-12-syllabus-adv-multivariate-esrm-6554/Lecture01/Lecture01.qmd

## Bayesian Analysis: Why It Is Used?

There are at least four main reasons why people use Bayesian Analysis:

1. Missing data
• Multiple imputation
• More complicated model for certain types of missing data
2. Lack of software capable of handing large sized analyses
• Have a zero-inflated Poisson model with 1000 observations and 1000 parameters? No problem in Bayesian!

## Bayesian Analysis: Why It Is Used? (Cont.)

1. New complex models not available in frequentist framework
• Have a new model? (A model that estimates the probability students choose the right answers then choose the wrong answers in a multiple choice test?)
2. Enjoy the Bayesian thinking process
• It is a way of thinking that everything is random and everything can be expressed as probability. It is a way of thinking that we can update our belief as we collect more data. It is a way of thinking that we can use our prior knowledge to help us understand the data.

## Bayesian Analysis: Issues

1. Subjective vs. Objective
• Prior distribution is subjective. It is based on your prior knowledge.
• However, 1) Scientific judgement is always subjective 2) you can use objective prior distribution to avoid this issue.
2. Computationally Intensive
• It is not a problem anymore. We have computers.
• But we still need weeks or months to get results for some complated model and big data
3. Difficult to understand

## What topics Bayesian Analysis can cover?

Funding available only for NIH, CDC, FDA, AHRQ, and ACF 2020 Spring. Source: https://report.nih.gov/

## Wrapping Up

1. We know what is “Bayesian” and its components.
2. Why Bayesian Estimation is different from other estimation, say maximum likelihood
3. We also know that Bayesian analysis is popular in many fields, especially complex data >>>>>>> 6e75efe (update 2024 CV):posts/2024-01-12-syllabus-adv-multivariate-esrm-6554/Lecture01/Lecture01.qmd

## Next Class

We will talk about how Bayesian methods works in a little bit more technical way.

## Suggestions

Your opinions are very important to me. Feel free to let me know if you have any suggestions on the course.