# Subjective, Objective, Assumption and Modeling

## Overview

There are 2 main methods to estimate parameters in statistics. i.e. frequency-based method (Maximum Likelihood Estimation) and Bayesian-based method (Bayesian Estimation). Frankly speaking, we are supposed to get a deep insight to it and have a intuitive understanding on estimation. In this note, I provisionally offer some dimensions to explore it. i.e. motivation, theoretical guarantee and algorithm. Maybe I will enrich it in my future study. Note that there is just a difference in modeling unknown thing between 2 methods, but both are statistical method, which aims to estimate the whole distribution from the sampling data (in some case, do hypothesis then verify it).

## Motivation

• frequency-based method is based on the assumption or belief that unknown thing is a fixed (or constant) value and we just don’t know it, but we are able to fit (or simulate) it by quantities of experiments (of course, more experiments, more confident). In other words, it is subjective and models unknown thing by uncertainty.

• Bayesian-based method is a application of Bayesian Formula in parameter estimation. Its key idea is that unknown thing is not fixed and obeys some distribution. Obviously, that distribution is also driven by some descriptive parameters (e.g. mean and variance). In other words, it is objective and models unknown thing by incompleteness (due to non full-fledged observations)

## Algorithm

• Given observations $D$ (i.i.d. experiments) and unknown parameter $\theta$, frequency-based method (MLE) maximizes the unknown belief from known experiments. That is, if it happens, it is more likely to happen once again. In detail, we maximize the probability of observed events $D$ conditioned on the parameter $\theta$.
$\theta^\star = \mathop{\arg\max}_\theta \{P(D|\theta)\}$
•  In the perspective of Bayesian-based method, our goal is not to find a fixed parameter. On the contrary, we need delineate the distribution behind the parameter, because of the aforementioned key idea of Bayesian-based method. Specifically, $\theta$ is driven by a distribution, namely **posterior distribution $P(\theta D)$. According to **Bayesian Formula:
$P(\theta|D) = \frac{P(D|\theta)P(\theta)}{P(D)} = \frac{P(D|\theta)P(\theta)}{\int_\theta P(D|\theta)P(\theta) d\theta}$

In the practical case, there are 2 kinds of approximation:

• In order to obtain a fixed value from a posterior distribution, we can force the posterior to be maximum (Maximum A Posterior), which is denoted as:

\begin{align} \theta^\star &= \mathop{\arg\max}_\theta \{P(\theta|D)\} \\ &= \mathop{\arg\max}_\theta\{ \frac{P(D|\theta)P(\theta)}{\int_\theta P(D|\theta)P(\theta) d\theta} \} \\ &= \mathop{\arg\max}_\theta\{P(D|\theta) P(\theta)\} \end{align}

Compared to MLE, the objective (after logarithm) has one more item than it. we can regard the extra item as regularization w.r.t. $\theta$ , which is from the prior distribution $P(\theta)$. Note that this kind of approximation is totally dependent on the assumption of prior.

•  A stronger approximation is that posterior has the same distribution as prior, namely Conjugate Distribution. The prior is regarded as conjugate prior w.r.t. likelihood $P(D \theta)$.

## Theoretical background

• MLE maximizes the belief, so it relies on experiments heavily. In other words, more experiments, more confident. (证明见：霍夫丁不等式， 大数定律)
• MAP is completely based on Bayesian Formula, which is just a classic viewpoint of Bayesian Party. (类似的：万物皆分布， 皆由分布背后的参数驱动， 例：贝叶斯神经网络)