bayesian_inference.Rmd

# (PART) 贝叶斯篇 {-}

# 贝叶斯推断 {#bayesian-inference}

```{r bayes-01, message=FALSE, warning=FALSE}
library(tidyverse)
library(tidybayes)
library(rstan)
library(brms)
rstan_options(auto_write = TRUE)
options(mc.cores = parallel::detectCores())
```


之前我们讲了线性模型和混合线性模型，今天我们往前一步，应该说是一大步。因为这一步迈向了贝叶斯分析，与频率学派的分析有本质的区别，这种区别类似经典物理和量子物理的区别。

- 频率学派，是从数据出发
- 贝叶斯。先假定参数有一个分布，看到数据后，再重新分配可能性。




> Statistical inference is the process of using observed data to infer properties of the statistical distributions that generated that data.




简单点说

$$
\Pr(\text{parameters} | \text{data}).
$$


这个量实际上贝叶斯定理中的后验概率分布(*posterior distribution*)

$$
\underbrace{\Pr(\text{parameters} | \text{data})}_{\text{posterior}} = \frac{\overbrace{\Pr(\text{data} | \text{parameters})}^{\text{likelihood}} \overbrace{\Pr(\text{parameters})}^{\text{prior}}}{\underbrace{\Pr(\text{data})}_{evidence}} .
$$

下面，通过具体的案例演示简单的贝叶斯推断(Bayesian inference)

## 学生身高的分布？

假定这是收集的200位学生身高和体重数据

```{r}
d <- readr::read_rds(here::here('demo_data', "height_weight.rds")) 
head(d)
```


用dplyr函数很容易得到样本的统计量

```{r}
d %>% 
  summarise(
    across(height, list(mean = mean, median = median, max = max, min = min, sd = sd))
)
```


```{r}
d %>% 
  ggplot(aes(x = height)) +
  geom_density()
```




## 推断

> 注意到，我们的数据只是样本，不代表全体分布。我们只有通过样本去**推断**全体分布情况。


通过前面的身高的统计量，我们可以合理的猜测：

- 均值可能是160，162，170，172，..., 或者说这个均值在一个范围之内，在这个范围内，有些值的可能性大，有些值可能性较低。比如，认为这值游离在(150,180)范围，其中168左右的可能最大，两端的可能性最低。如果寻求用数学语言来描述，它符合正态分布的特征

- 方差也可以假设在(0, 50)范围内都有可能，而且每个位置上的概率都相等



把我们的猜测画出来就是这样的，

```{r, fig.width = 6, fig.height = 2.5}
library(patchwork)
p1 <- 
  ggplot(data = tibble(x = seq(from = 100, to = 230, by = .1)), 
       aes(x = x, y = dnorm(x, mean = 168, sd = 20))) +
  geom_line() +
  xlab("height_mean") +
  ylab("density")


p2 <- 
  ggplot(data = tibble(x = seq(from = -10, to = 55, by = .1)), 
       aes(x = x, y =  dunif(x, min  = 0,   max = 50))) +
  geom_line() +
  xlab("height_sd") +
  ylab("density")

p1 + p2
```


### 参数空间

我们这里构建 1000*1000个 (`mu, sigma`) 参数空间


```{r}
d_grid <- crossing(
     mu = seq(from = 150, to = 190, length.out = 1000),
  sigma = seq(from = 4,   to = 9,   length.out = 1000)
)

d_grid
```





### likelihood

参数空间里，计算在每个(mu, sigma)组合下，身高值(`d$height`)出现的概率密度`dnorm(d2$height, mean = mu, sd = sigma)`，然后加起来。
很显然，不同的(mu, sigma)，概率密度之和是不一样的，我们这里有1000*1000 个(mu, sigma)组合，
所以会产生 1000*1000 个值



```{r}
grid_function <- function(mu, sigma) {
	dnorm(d$height, mean = mu, sd = sigma, log = T) %>% 
		sum()
}
```




```{r, eval=FALSE}
d_grid %>% 
	mutate(log_likelihood = map2_dbl(mu, sigma, grid_function)) 
```






### prior


```{r, eval=FALSE}
d_grid %>% 
	mutate(prior_mu     = dnorm(mu,    mean = 178, sd  = 20, log = T),
	     prior_sigma    = dunif(sigma, min  = 0,   max = 50, log = T)) 
```




### posterior


```{r}
d_grid <-
	d_grid %>%
	mutate(log_likelihood = map2_dbl(mu, sigma, grid_function)) %>%
	mutate(prior_mu       = dnorm(mu,    mean = 168, sd  = 20, log = T),
	       prior_sigma    = dunif(sigma, min  = 0,   max = 50, log = T)) %>%
	mutate(product        = log_likelihood + prior_mu + prior_sigma) %>%
	mutate(probability    = exp(product - max(product)))

head(d_grid)
```




```{r, fig.width = 3.25, fig.height = 3}
d_grid %>%
  ggplot(aes(x = mu, y = sigma, z = probability)) +
  geom_contour() +
  labs(
    x = expression(mu),
    y = expression(sigma)
  ) +
  coord_cartesian(
    xlim = range(d_grid$mu),
    ylim = range(d_grid$sigma)
  ) +
  theme(panel.grid = element_blank())
```


```{r, fig.width = 4.5, fig.height = 3}
d_grid %>%
  ggplot(aes(x = mu, y = sigma)) +
  geom_raster(
    aes(fill = probability),
    interpolate = T
  ) +
  scale_fill_viridis_c(option = "A") +
  labs(
    x = expression(mu),
    y = expression(sigma)
  ) +
  theme(panel.grid = element_blank())
```





### sampling from posterior

后验分布按照probability值的大小来抽样。

```{r}
d_grid_samples <- 
	d_grid %>% 
	sample_n(size = 1e4, replace = T, weight = probability)
```


```{r, fig.width = 3.25, fig.height = 3}
d_grid_samples %>% 
	ggplot(aes(x = mu, y = sigma)) + 
	geom_point(size = .9, alpha = 1/15) +
	scale_fill_viridis_c() +
	labs(x = expression(mu[samples]),
		 y = expression(sigma[samples])) +
	theme(panel.grid = element_blank())
```



```{r, fig.width = 6, fig.height = 3}
d_grid_samples %>%
	select(mu, sigma) %>%
	pivot_longer(
	  cols = everything(),
	  names_to = "key",
	  values_to = "value"
	) %>%
	ggplot(aes(x = value)) +
	geom_density(fill = "grey33", size = 0) +
	scale_y_continuous(NULL, breaks = NULL) +
	xlab(NULL) +
	theme(panel.grid = element_blank()) +
	facet_wrap(~key, scales = "free")
```




### 最高密度区间

也可以用`tidybayes::mode_hdi()`得到后验概率的**最高密度区间**

```{r}
library(tidybayes)

d_grid_samples %>%
	select(mu, sigma) %>%
	pivot_longer(
	  cols = everything(),
	  names_to = "key",
	  values_to = "value"
	) %>%
	group_by(key) %>%
	mode_hdi(value)
```



以上是通过**网格近似**的方法得到height分布的后验概率，但这种方法需要构建参数网格，对于较复杂的模型，计算量会陡增，内存占用大、比较费时，因此在实际的数据中，一般不采用这种方法，但网格近似的方法可以帮助我们很好地理解贝叶斯数据分析。





## 参考资料

- https://mc-stan.org/
- https://github.com/jgabry/bayes-workflow-book
- https://github.com/XiangyunHuang/masr/
- https://github.com/ASKurz/Statistical_Rethinking_with_brms_ggplot2_and_the_tidyverse_2_ed/
- 《Regression and Other Stories》, Andrew Gelman, Cambridge University Press. 2020
- 《A Student's Guide to Bayesian Statistics》,  Ben Lambert, 2018
- 《Statistical Rethinking: A Bayesian Course with Examples in R and STAN》 ( 2nd Edition), by Richard McElreath, 2020
- 《Bayesian Data Analysis》, Third Edition, 2013
- 《Doing Bayesian Data Analysis: A Tutorial with R, JAGS, and Stan》  (2nd Edition) John Kruschke, 2014
- 《Bayesian Models for Astrophysical Data: Using R, JAGS, Python, and Stan》， Joseph M. Hilbe, Cambridge University Press, 2017


```{r, echo = F, message = F, warning = F, results = "hide"}
pacman::p_unload(pacman::p_loaded(), character.only = TRUE)
```