tidystats_t_test.Rmd

# 双样本t检验 {#tidystats-t-test}

本章需要的宏包，希望大家提前安装

```{r, eval=FALSE}
install.packages(c("bayesplot", "palmerpenguins", "rstatix", "broom", "ggstatsplot", "infer", "ggthemes"))
```



## 实验设计

研究某种药物的疗效，一般采用**大样本随机双盲对照试验**，**比较**在特定条件下被试的反应，获取相关数据后，会进行组内比较或者组间比较：

- **组内比较**， 同一组人，每个人要完成多次测量（重复测量），比如服药第一天的情况，服药第二天的情况，服药第三天的情况...，每组的人数是恒定的。

- **组间比较**，`A`组的被试吃1mg，`B`组被试吃2mg, `C`组吃3mg...，每组的人数不要求是恒定的。


这个过程可能会使用`two sample t-tests`。



## 提问

我们以企鹅体征数据作为案例，假定企鹅就是我们的被试

```{r, message=FALSE, warning=FALSE}
library(tidyverse)
theme_set(bayesplot::theme_default())

penguins <- palmerpenguins::penguins %>%
  drop_na()
```

提出问题：

- 企鹅有男女两种性别(`female, male`)，不同性别的`bill_length_mm`的均值是否相同？

- 企鹅种类有三种(`Adelie, Chinstrap, Gentoo`)，比较在每个种类下男企鹅和女企鹅`bill_length_mm`的均值？

- 两两比较不同种类的`bill_length_mm`的均值？



### 不同性别的嘴峰长度的均值是否相同

强烈推荐大家先可视化探索
```{r}
penguins %>%
  ggplot(aes(x = sex, y = bill_length_mm)) +
  geom_boxplot() +
  geom_jitter() +
  theme(legend.position = "none")
```


接着简单计算，不同性别`bill_length_mm`均值以及差值
```{r}
penguins %>%
  group_by(sex) %>%
  summarize(avg_rating = mean(bill_length_mm, na.rm = TRUE)) %>%
  mutate(diff_means = avg_rating - lag(avg_rating))
```


#### using `t.test()`

```{r}
t.test(
  bill_length_mm ~ sex, 
  data = penguins, 
  var.equal = TRUE    # `var.equal = ` 假定两个样本方差是否相等
)
```


```{r}
t.test(
  bill_length_mm ~ sex, 
  data = penguins, 
  var.equal = TRUE
) %>%
  broom::tidy()
```


#### using `rstatix::t_test()`

`rstatix`宏包提供了类似`dplyr`风格的语法

```{r}
library(rstatix)

penguins %>%
  rstatix::t_test(bill_length_mm ~ sex, var.equal = TRUE)
```



#### using `ggstatsplot::ggbetweenstats()` 

探索性数据分析，将包含数据可视化和统计建模两个阶段，可视化为建模提供依据，模型反过来又可以提出不同的可视化方法。`ggstatsplot`将这两个阶段统一在图形中，即绘制带有统计检验信息的图形，提高数据探索的速度和效率。

```{r}
library(ggstatsplot)

penguins %>% 
  ggbetweenstats( 
    x = sex,
    y = bill_length_mm,
    pairwise.comparisons = TRUE,
    pairwise.display = "all",
    var.equal = TRUE
  )
```



#### using `infer`: 基于模拟的检验

```{r,  echo = FALSE, fig.cap = "Hypothesis Testing Framework"}
knitr::include_graphics("images/downey.png")
```


-  实际观察的差别

```{r}
library(infer)
obs_diff <- penguins %>%
  specify(formula = bill_length_mm ~ sex) %>%
  calculate(
    stat = "diff in means",
    order = c("male", "female")
  )
obs_diff
```

- 模拟

```{r}
null_dist <- penguins %>%
  specify(formula = bill_length_mm ~ sex) %>%
  hypothesize(null = "independence") %>%
  generate(reps = 5000, type = "permute") %>% 
  calculate(
    stat = "diff in means",
    order = c("male", "female")
  )
head(null_dist)
```


::: {.rmdnote}
1. `specify()` 指定解释变量和被解释变量 (`y ~ x`)

2. `hypothesize()` 指定**零假设** (比如, `independence`= `y` 和 `x` 彼此独立)

3. `generate()` 从基于零假设的平行世界中抽样:
    - `reps`，指定抽样次数
    - `type`，指定重抽样的类型。

4. `calculate()` 计算每次抽样的统计值 (`stat = "diff in means"`)

::: 




- 可视化

```{r}
null_dist %>%
  visualize() +
  shade_p_value(obs_stat = obs_diff, direction = "both")
```

- 计算p值

```{r}
pvalue <- null_dist %>%
  get_pvalue(obs_stat = obs_diff, direction = "two_sided")

pvalue
```



#### using `lm()`
```{r}
model <- lm(bill_length_mm ~ sex, data = penguins)
broom::tidy(model)
```

```{r}
confint(model)
```
可以看到，95%的置信区间与用`t.test()`的结果完全一样。




### 每个种类下男企鹅和女企鹅`bill_length_mm`的均值

企鹅种类有三种，比较在每个种类下男企鹅和女企鹅`bill_length_mm`的均值？意思是**多次t-test**

```{r}
penguins %>%
  ggplot(aes(x = species, y = bill_length_mm, color = sex)) +
  geom_boxplot(position = position_dodge(0.8)) +
  geom_jitter(
    position = position_jitterdodge()
  ) +
  scale_x_discrete(
    expand = expansion(mult = c(0.3, 0.3))
  ) +
  theme(legend.position = "none")
```


#### using `group_modify() + t.test()`

```{r}
penguins %>%
  group_by(species) %>%
  group_modify(
    ~ t.test(bill_length_mm ~ sex, data = .x, var.equal = TRUE) %>%
      broom::tidy()
  )
```


#### using `rstatix::t_test()`

```{r}
library(rstatix)

penguins %>%
  group_by(species) %>%
  rstatix::t_test(bill_length_mm ~ sex, var.equal = TRUE)
```


#### using `ggstatsplot::grouped_ggbetweenstats()` 

```{r, fig.width = 12, fig.asp = 0.618}
library(ggstatsplot)

penguins %>% 
  grouped_ggbetweenstats(
    x = sex,
    y = bill_length_mm,
    pairwise.comparisons = TRUE,
    pairwise.display = "all",
    var.equal = TRUE,
    grouping.var = species  # group
  )
```





### 两两比较不同种类的`bill_length_mm`的均值

企鹅种类有三种，两两比较不同种类的`bill_length_mm`的均值。
  
- `Adelie    - Chinstrap`
- `Adelie    - Gentoo`
- `Chinstrap - Gentoo`


```{r, error=TRUE}
t.test(bill_length_mm ~ species, data = penguins) 
```

species 有三组，也就说有三个层级，程序不接受。方法是：**成对pairwise t-tests**



#### using `pairwise.t.test()`

```{r, eval=FALSE}
pairwise.t.test(x, y) # x is a vector of the data, y is the group factor
```


```{r}
pairwise.t.test(
  penguins$bill_length_mm, penguins$species,
  alternative = "two.sided",
  paired = FALSE,     
  p.adj = "holm"
) %>%
  broom::tidy()
```


::: {.rmdnote}

注意：pairwise t-tests并不是简单地把每一个可能的配对都做一次t-test

```{r}
penguins %>%
  filter(species %in% c("Gentoo", "Chinstrap")) %>%
  t.test(bill_length_mm ~ species, data = .) %>%
  broom::tidy()
```
:::




#### using `rstatix::pairwise_t_test()`

```{r}
library(rstatix)

penguins %>%
  pairwise_t_test(
    bill_length_mm ~ species,
    p.adjust.method = "holm",
    alternative = "two.sided",
    paired = FALSE 
  )
```



#### using `ggstatsplot::ggbetweenstats()` 

```{r, fig.height = 5}
penguins %>%
  ggstatsplot::ggbetweenstats( 
    x = species, 
    y = bill_length_mm,
    pairwise.comparisons = TRUE,
    pairwise.display = "all",
    p.adjust.method = "holm",
    messages = FALSE,
    var.equal = TRUE,
    alternative = "two.sided",
    ggtheme = ggthemes::theme_economist(),
    package = "wesanderson",
    palette = "Darjeeling1"
  )
```





## 参考

- <https://github.com/kassambara/rstatix>
- <https://github.com/IndrajeetPatil/ggstatsplot>
- <http://allendowney.blogspot.com/2016/06/there-is-still-only-one-test.html>
- <https://infer.netlify.app/articles/t_test.html>




```{r, echo = F}
# remove the objects
# rm(list=ls())
rm(penguins, model, obs_diff, null_dist, pvalue)
```

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