```{r}
setwd("/Users/arthurdanjou/Workspace/studies/M1/General Linear Models/TP3")
```

# Question 1 : Import dataset and check variables

```{r}
library(dplyr)
ozone <- read.table("ozone.txt", header = TRUE, sep = " ", dec = ".")
ozone$vent <- as.factor(ozone$vent)
ozone$temps <- as.factor(ozone$temps)
ozone <- ozone |> mutate(across(where(is.character), as.numeric))
ozone <- ozone |> mutate(across(where(is.integer), as.numeric))
paged_table(ozone)
```

# Question 2 : max03 ~ T12

```{r}
model_T12 <- lm(maxO3 ~ T12, data = ozone)
summary(model_T12)
```

```{r}
library(ggplot2)

ggplot(ozone, aes(x = T12, y = maxO3)) +
  geom_smooth(method = "lm", se = T) +
  geom_point(col = "red", size = 0.5) +
  labs(title = "maxO3 ~ T12") +
  theme_minimal()
```

```{r}
library(ggfortify)
library(car)

autoplot(model_T12, 1:4)
durbinWatsonTest(model_T12)
```

The p-value of the Durbin-Watson test is 0, so [P3] is not valid. The residuals are correlated.

The model is not valid.

# Question 3 : max03 ~ T12 + temps

```{r}
library(ggplot2)

ggplot(ozone, aes(y = maxO3, x = temps, colour = temps, fill = temps)) +
  geom_boxplot(alpha = 0.5, outlier.alpha = 0) +
  geom_jitter(width = 0.25, size = 1) +
  stat_summary(fun = mean, colour = "black", geom = "point", shape = 18, size = 3)
```

```{r}
model_temps <- lm(maxO3 ~ -1 + temps, data = ozone)
summary(model_temps)
```

```{r}
autoplot(model_temps, 1:4)
durbinWatsonTest(model_temps)
```
```{r}
model_vent <- lm(maxO3 ~ -1 + vent, data = ozone)
summary(model_vent)
```

```{r}
autoplot(model_vent, 1:4)
durbinWatsonTest(model_vent)
```
The p-value of the Durbin-Watson test is 0, so [P3] is not valid. The residuals are correlated.

The model is not valid.
```{r}
model_temps_vent <- lm(maxO3 ~ temps * vent, data = ozone)
summary(model_temps_vent)
```

```{r}
autoplot(model_temps_vent, 1:4)
durbinWatsonTest(model_temps_vent)
```

# Question 4 : Multiple linear regression
```{r}
model <- lm(maxO3 ~ ., data = ozone)
summary(model)
```

```{r}
autoplot(model, 1:4)
durbinWatsonTest(model)
```

[P1] is verified as the 'Residuals vs Fitted' plot shows that the points are well distributed around 0
[P2] is verified as the 'Scale-Location' plot shows that the points are well distributed around 1
[P4] is verified as the 'QQPlot' is aligned with the 'y=x' line

[P3] is verified as the p-value is 0.7 > 0.05, so we do not reject H0 so the residuals are not auto-correlated

The model is valid.

```{r}
library(GGally)
ggpairs(ozone, progress = FALSE)
```
```{r}
library(MASS)

model_backward <- stepAIC(model, ~., trace = FALSE, direction = c("backward"))
summary(model_backward)
```

```{r}
AIC(model_backward)
autoplot(model_backward, 1:4)
durbinWatsonTest(model_backward)
```

# Question 5 : Prediction
```{r}
new_obs <- list(
  T12 = 18,
  Ne9 = 3,
  Vx9 = 0.7,
  maxO3v = 85
)
predict(model_backward, new_obs, interval = "confidence")
```