---
title: "Regression and Other Stories: Robit"
author: "Andrew Gelman, Jennifer Hill, Aki Vehtari"
date: "`r format(Sys.Date())`"
output:
  html_document:
    theme: readable
    toc: true
    toc_depth: 2
    toc_float: true
    code_download: true
---
Comparison of robit and logit models for binary data. See Chapter
15 in Regression and Other Stories.

-------------


```{r setup, include=FALSE}
knitr::opts_chunk$set(message=FALSE, error=FALSE, warning=FALSE, comment=NA)
# switch this to TRUE to save figures in separate files
savefigs <- FALSE
```

#### Load packages

```{r }
library("rprojroot")
root<-has_file(".ROS-Examples-root")$make_fix_file()
library("cmdstanr")
options(mc.cores = 1)
library("ggdist")
logit <- qlogis
invlogit <- plogis
```

## Generate data from logit model

```{r }
# set the random seed to get reproducible results
# change the seed to experiment with variation due to random noise
set.seed(1234)
N <- 50
x <- runif(N, -9, 9)
a <- 0
b <- 0.8
p <- invlogit(a + b*x)
y <- rbinom(N, 1, p)
df <- 4
data_1 <- list(N=N, x=x, y=y, df=df)
```

## Fit logit and probit models using the simulated data

#### Show Stan code for the models

```{r }
writeLines(readLines(root("Robit","logit.stan")))
writeLines(readLines(root("Robit","robit.stan")))
```

#### Compile models

```{r }
logit_model <- cmdstan_model("logit.stan")
robit_model <- cmdstan_model("robit.stan")
```

#### Sample and compute posterior medians

```{r }
fit_logit_1 <- logit_model$sample(data=data_1, refresh=0)
print(fit_logit_1)
a_hat_logit_1 <- median(fit_logit_1$draws("a"))
b_hat_logit_1 <- median(fit_logit_1$draws("b"))
```

#### Sample and compute posterior medians

```{r }
fit_robit_1 <- robit_model$sample(data=data_1, refresh=0)
print(fit_robit_1)
a_hat_robit_1 <- median(fit_robit_1$draws("a"))
b_hat_robit_1 <- median(fit_robit_1$draws("b"))
```

#### Plot

```{r }
if (savefigs) pdf("logistic2b.pdf", height=4, width=6)
```
```{r }
par(mar=c(3,3,2,1), mgp=c(1.5,.5,0), tck=-.01)
plot(data_1$x, data_1$y, yaxt="n", main="Data from a logistic regression", xlab="x", ylab="y")
axis(2, c(0,1))
curve(invlogit(a_hat_logit_1 + b_hat_logit_1*x), add=TRUE, lty=2)
curve(pstudent_t(a_hat_robit_1 + b_hat_robit_1*x, data_1$df, 0, sqrt((data_1$df-2)/data_1$df)), add=TRUE, lty=1)
legend (1, .3, c("fitted logistic regression", "fitted robit regression"), lty=c(2,1), cex=.8)
```
```{r eval=FALSE, include=FALSE}
if (savefigs) dev.off()
```

## Add an outlier by flipping the class of one observation

```{r }
low_value <- (1:N)[x==sort(x)[4]]
data_2 <- data_1
data_2$y[low_value] <- 1
```

#### Sample and compute posterior medians

```{r }
fit_logit_2 <- logit_model$sample(data=data_2, refresh=0)
print(fit_logit_2)
a_hat_logit_2 <- median(fit_logit_2$draws("a"))
b_hat_logit_2 <- median(fit_logit_2$draws("b"))
```

#### Sample and compute posterior medians

```{r }
fit_robit_2 <- robit_model$sample(data=data_2, refresh=0)
print(fit_robit_2)
a_hat_robit_2 <- median(fit_robit_2$draws("a"))
b_hat_robit_2 <- median(fit_robit_2$draws("b"))
```

Plot

```{r }
if (savefigs) pdf("logistic2a.pdf", height=4, width=6)
```
```{r }
par(mar=c(3,3,2,1), mgp=c(1.5,.5,0), tck=-.01)
plot(data_2$x, data_2$y, yaxt="n", main="Contaminated data", xlab="x", ylab="y")
axis(2, c(0,1))
curve(invlogit(a_hat_logit_2 + b_hat_logit_2*x), add=TRUE, lty=2)
curve(pstudent_t(a_hat_robit_2 + b_hat_robit_2*x, data_2$df, 0, sqrt((data_2$df-2)/data_2$df)), add=TRUE, lty=1)
legend (1, .3, c("fitted logistic regression", "fitted robit regression"), lty=c(2,1), cex=.8)
```
```{r eval=FALSE, include=FALSE}
if (savefigs) dev.off()
```

