31-rda.Rmd

# Regularized Discriminant Analysis

We now use the  `Sonar` dataset from the `mlbench` package to explore a new regularization method, **regularized discriminant analysis** (RDA), which combines the LDA and QDA. This is similar to how elastic net combines the ridge and lasso.

## Sonar Data

```{r}
# this is a temporary workaround for an issue with glmnet, Matrix, and R version 3.3.3
# see here: http://stackoverflow.com/questions/43282720/r-error-in-validobject-object-when-running-as-script-but-not-in-console
library(methods)
```

```{r, message = FALSE, warning = FALSE}
library(mlbench)
library(caret)
library(glmnet)
library(klaR)
```

```{r}
data(Sonar)
```

```{r}
#View(Sonar)
```

```{r}
table(Sonar$Class) / nrow(Sonar)
```

```{r}
ncol(Sonar) - 1
```

## RDA

Regularized discriminant analysis uses the same general setup as LDA and QDA but estimates the covariance in a new way, which combines the covariance of QDA $(\hat{\Sigma}_k)$ with the covariance of LDA $(\hat{\Sigma})$ using a tuning parameter $\lambda$.

$$
\hat{\Sigma}_k(\lambda) = (1-\lambda)\hat{\Sigma}_k + \lambda \hat{\Sigma}
$$

Using the `rda()` function from the `klaR` package, which `caret` utilizes, makes an additional modification to the covariance matrix, which also has a tuning parameter $\gamma$.

$$
\hat{\Sigma}_k(\lambda,\gamma) = (1 -\gamma) \hat{\Sigma}_k(\lambda) + \gamma \frac{1}{p} \text{tr}(\hat{\Sigma}_k(\lambda)) I
$$

Both $\gamma$ and $\lambda$ can be thought of as mixing parameters, as they both take values between 0 and 1. For the four extremes of $\gamma$ and $\lambda$, the covariance structure reduces to special cases:

- $(\gamma=0, \lambda=0)$: QDA - individual covariance for each group.
- $(\gamma=0, \lambda=1)$: LDA - a common covariance matrix.
- $(\gamma=1, \lambda=0)$: Conditional independent variables - similar to Naive Bayes, but variable variances within group (main diagonal elements) are all equal.
- $(\gamma=1, \lambda=1)$: Classification using euclidean distance - as in previous case, but variances are the same for all groups. Objects are assigned to group with nearest mean.


## RDA with Grid Search

```{r}
set.seed(1337)
cv_5_grid = trainControl(method = "cv", number = 5)
```

```{r}
set.seed(1337)
fit_rda_grid = train(Class ~ ., data = Sonar, method = "rda", trControl = cv_5_grid)
fit_rda_grid
```

```{r}
plot(fit_rda_grid)
```

## RDA with Random Search


```{r}
set.seed(1337)
cv_5_rand = trainControl(method = "cv", number = 5, search = "random")
```

```{r}
fit_rda_rand = train(Class ~ ., data = Sonar, method = "rda",
                     trControl = cv_5_rand, tuneLength = 9)
fit_rda_rand
```

```{r}
ggplot(fit_rda_rand)
```


## Comparison to Elastic Net

```{r}
set.seed(1337)
fit_elnet_grid = train(Class ~ ., data = Sonar, method = "glmnet",
                       trControl = cv_5_grid, tuneLength = 10)
```

```{r}
set.seed(1337)
fit_elnet_int_grid = train(Class ~ . ^ 2, data = Sonar, method = "glmnet",
                           trControl = cv_5_grid, tuneLength = 10)
```


## Results

```{r}
get_best_result = function(caret_fit) {
  best_result = caret_fit$results[as.numeric(rownames(caret_fit$bestTune)), ]
  rownames(best_result) = NULL
  best_result
}
```

```{r}
knitr::kable(rbind(
  get_best_result(fit_rda_grid),
  get_best_result(fit_rda_rand)))
```

```{r}
knitr::kable(rbind(
  get_best_result(fit_elnet_grid),
  get_best_result(fit_elnet_int_grid)))

```


## External Links

- [Random Search for Hyper-Parameter Optimization](http://www.jmlr.org/papers/volume13/bergstra12a/bergstra12a) - Paper justifying random tuning parameter search.
- [Random Hyperparameter Search](https://topepo.github.io/caret/random-hyperparameter-search.html) - Details on random tuning parameter search in `caret`.


## RMarkdown

The RMarkdown file for this chapter can be found [**here**](31-rda.Rmd). The file was created using `R` version `r paste0(version$major, "." ,version$minor)` and the following packages:

- Base Packages, Attached

```{r, echo = FALSE}
sessionInfo()$basePkgs
```

- Additional Packages, Attached

```{r, echo = FALSE}
names(sessionInfo()$otherPkgs)
```

- Additional Packages, Not Attached

```{r, echo = FALSE}
names(sessionInfo()$loadedOnly)
```