東京大学 情報理工学系研究科 コンピュータ科学専攻 2019年8月実施 専門科目II 問題6
Author
Description
The probability density function of the normal distribution
Let
Answer the following questions.
(1) Express the expectation
(2) Show that the conditional distribution of
(3) Let
(4) Consider maximum-likelihood estimation of
where
- (i) Express
using and . - (ii) Express
using and .
Kai
(1)
The random variable
- Expectation of
:
- Variance of
:
(2)
To find the conditional distribution of
- Expectation of
:
- Variance of
:
This can be derived using the properties of conditional distributions for bivariate normal distributions.
(3)
The joint probability density function
Expanding this, we get:
(4)
(i)
The expectation
Simplifying further using the properties of the expectation for a normal distribution:
(ii)
The update rule for
Solving this for
This update rule depends on the observed data
Knowledge
正态分布 条件分布 数值期望 EM算法 最大似然估计
难点思路
推导条件分布涉及到二元正态分布的性质,尤其是推导条件期望和方差时,需要对协方差矩阵有深刻理解。EM 算法的难点在于构建对数似然函数的期望,并通过优化找到参数的更新规则。
解题技巧和信息
- 条件分布:对于二元正态分布,条件分布仍然是正态分布,且其参数可以通过边际分布的参数计算得到。
- EM 算法:EM 算法通过最大化对数似然函数的期望来迭代更新参数,对于缺失数据的问题尤为有效。
- 最大似然估计:通常情况下,EM 算法能够保证参数的渐进一致性,即经过多次迭代,参数估计会收敛到真值。
重点词汇
- Expectation-Maximization (EM) Algorithm: 期望最大化算法
- Conditional distribution: 条件分布
- Maximum likelihood estimation: 最大似然估计
- Normal distribution: 正态分布
参考资料
- Bishop, C. M. (2006). Pattern Recognition and Machine Learning. Springer. Chapter 9: Mixture Models and EM.
- Casella, G., & Berger, R. L. (2001). Statistical Inference (2nd ed.). Duxbury. Chapter 7: Estimation.