21-caret.Rmd

# The `caret` Package

```{r caret_opts, include = FALSE}
knitr::opts_chunk$set(cache = TRUE, autodep = TRUE, fig.align = "center")
```

Now that we have seen a number of classification and regression methods, and introduced cross-validation, we see the general outline of a predictive analysis:

- Test-train split the available data
    - Consider a method
        - Decide on a set of candidate models (specify possible tuning parameters for method)
        - Use resampling to find the "best model" by choosing the values of the tuning parameters
    - Use chosen model to make predictions
    - Calculate relevant metrics on the test data

At face value it would seem like it should be easy to repeat this process for a number of different methods, however we have run into a number of difficulties attempting to do so with `R`.

- The `predict()` function seems to have a different behavior for each new method we see.
- Many methods have different cross-validation functions, or worse yet, no built-in process for cross-validation.
- Not all methods expect the same data format. Some methods do not use formula syntax.
- Different methods have different handling of categorical predictors. Some methods cannot handle factor variables.

Thankfully, the `R` community has essentially provided a silver bullet for these issues, the [`caret`](http://topepo.github.io/caret/) package. Returning to the above list, we will see that a number of these tasks are directly addressed in the `caret` package.

- Test-train split the available data
    - `createDataPartition()` will take the place of our manual data splitting. It will also do some extra work to ensure that the train and test samples are somewhat similar.
- Specify possible tuning parameters for method
    - `expand.grid()` is not a function in `caret`, but we will get in the habit of using it to specify a grid of tuning parameters.
- Use resampling to find the "best model" by choosing the values of the tuning parameters
    - `trainControl()` will specify the resampling scheme
    - `train()` is the workhorse of `caret`. It takes the following information then trains (tunes) the requested model:
        - `form`, a formula, such as `y ~ .`
            - This specifies the response and which predictors (or transformations of) should be used.
        - `data`, the data used for training
        - `trControl` which specifies the resampling scheme, that is, how cross-validation should be performed to find the best values of the tuning parameters
        - `preProcess` which allows for specification of data pre-processing such as centering and scaling
        - `method`, a statistical learning method from [a long list of available models](https://topepo.github.io/caret/available-models.html)
        - `tuneGrid` which specifies the tuning parameters to train over
- Use chosen model to make predictions
    - `predict()` used on objects of type `train` will be truly magical!


## Classification

To illustrate `caret`, first for classification, we will use the `Default` data from the `ISLR` package.

```{r}
data(Default, package = "ISLR")
```

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

We first test-train split the data using `createDataPartition`. Here we are using 75% of the data for training.

```{r}
set.seed(430)
default_idx = createDataPartition(Default$default, p = 0.75, list = FALSE)
default_trn = Default[default_idx, ]
default_tst = Default[-default_idx, ]
```

At first glance, it might appear as if the use of `createDataPartition()` is no different than our previous use of `sample()`. However, `createDataPartition()` tries to ensure a split that has a similar distribution of the supplied variable in both datasets. See the documentation for details.

After splitting the data, we can begin training a number of models. We begin with a simple additive logistic regression.

```{r}
default_glm_mod = train(
  form = default ~ .,
  data = default_trn,
  trControl = trainControl(method = "cv", number = 5),
  method = "glm",
  family = "binomial"
)
```

Here, we have supplied four arguments to the `train()` function form the `caret` package.

- `form = default ~ .` specifies the `default` variable as the response. It also indicates that all available predictors should be used.
- `data = default_trn` specifies that training will be down with the `default_trn` data
- `trControl = trainControl(method = "cv", number = 5)` specifies that we will be using 5-fold cross-validation.
- `method = glm` specifies that we will fit a generalized linear model.

The `method` essentially specifies both the model (and more specifically the function to fit said model in `R`) and package that will be used. The `train()` function is essentially a wrapper around whatever method we chose. In this case, the function is the base `R` function `glm()`, so no additional package is required. When a method requires a function from a certain package, that package will need to be installed. See the [list of available models](https://topepo.github.io/caret/available-models.html) for package information.

The list that we have passed to the `trControl` argument is created using the `trainControl()` function from `caret`. The `trainControl()` function is a powerful tool for specifying a number of the training choices required by `train()`, in particular the resampling scheme. 

```{r}
trainControl(method = "cv", number = 5)[1:3]
```

Here we see just the first three elements of this list, which are related to how the resampling will be done. These are the three elements that we will be most interested in. Here, only the first two are relevant.

- `method` specifies how resampling will be done. Examples include `cv`, `boot`, `LOOCV`, `repeatedcv`, and `oob`.
- `number` specifies the number of times resampling should be done for methods that require resample, such as, `cv` and `boot`.
- `repeats` specifies the number of times to repeat resampling for methods such as `repeatedcv`

For details on the full capabilities of this function, see the relevant documentation. The out-of-bag, `oob` which is a sort of automatic resampling for certain statistical learning methods, will be introduced later.

We've also passed an additional argument of `"binomial"` to `family`. This isn't actually an argument for `train()`, but an additional argument for the method `glm`. In actuality, we don't need to specify the family. Since `default` is a factor variable, `caret` automatically detects that we are trying to perform classification, and would automatically use `family = "binomial"`. This isn't the case if we were simply using `glm()`.

```{r}
default_glm_mod
```

Called the stored `train()` object summarizes the training that we have done. We see that we used 7501 observations that had a binary class response and three predictors. We have not done any data pre-processing, and have utilized 5-fold cross-validation. The cross-validated accuracy is reported. Note that, `caret` is an optimist, and prefers to report accuracy (proportion of correct classifications) instead of the error that we often considered before (proportion of incorrect classifications).

```{r}
names(default_glm_mod)
```

We see that there is a wealth of information stored in the list returned by `train()`. Two elements that we will often be interested in are `results` and `finalModel`.

```{r}
default_glm_mod$results
```

The `results` show some more detailed results, in particular `AccuracySD` which gives us an estimate of the uncertainty in our accuracy estimate.

```{r}
default_glm_mod$finalModel
```

The `finalModel` is a model object, in this case, the object returned from `glm()`. This final model, is fit to all of the supplied training data. This model object is often used when we call certain relevant functions on the object returned by `train()`, such as `summary()`

```{r}
summary(default_glm_mod)
```

We see that this summary is what we had seen previously from objects of type `glm`.

```{r}
calc_acc = function(actual, predicted) {
  mean(actual == predicted)
}
```

To obtain test accuracy, we will need to make predictions on the test data. With the object returned by `train()`, this is extremely easy.

```{r}
head(predict(default_glm_mod, newdata = default_tst))
```

We see that by default, the `predict()` function is returning classifications. This will be true no matter what method we use!

```{r}
# test acc
calc_acc(actual = default_tst$default,
         predicted = predict(default_glm_mod, newdata = default_tst))
```

If instead of the default behavior of returning classifications, we instead wanted predicted probabilities, we simply specify `type = "prob"`.

```{r}
# get probs
head(predict(default_glm_mod, newdata = default_trn, type = "prob"))
```

Notice that this returns the probabilities for all possible classes, in this case `No` and `Yes`. Again, this will be true for all methods! This is especially useful for multi-class data!.

### Tuning

Since logistic regression has no tuning parameters, we haven't really highlighted the full potential of `caret`. We've essentially used it to obtain cross-validated results, and for the more well-behaved `predict()` function. These are excellent improvements over our previous methods, but the real power of `caret` is its ability to provide a framework for tuning model.

To illustrate tuning, we now use `knn` as our method, which performs $k$-nearest neighbors.

```{r}
default_knn_mod = train(
  default ~ .,
  data = default_trn,
  method = "knn",
  trControl = trainControl(method = "cv", number = 5)
)
```

First, note that we are using formula syntax here, where previously we needed to create separate response and predictors matrices. Also, we're using a factor variable as a predictor, and `caret` seems to be taking care of this automatically.

```{r}
default_knn_mod
```

Here we are again using 5-fold cross-validation and no pre-processing. Notice that we now have multiple results, for `k = 5`, `k = 7`, and `k = 9`.

Let's modifying this training by introducing pre-processing, and specifying our own tuning parameters, instead of the default values above.

```{r}
default_knn_mod = train(
  default ~ .,
  data = default_trn,
  method = "knn",
  trControl = trainControl(method = "cv", number = 5),
  preProcess = c("center", "scale"),
  tuneGrid = expand.grid(k = seq(1, 101, by = 2))
)
```

Here, we've specified that we would like to center and scale the data. Essentially transforming each predictor to have mean 0 and variance 1. The documentation on the `preProcess()` function provides examples of additional possible pre-processing. IN our call to `train()` we're essentially specifying how we would like this function applied to our data.

We've also provided a "tuning grid," in this case, the values of `k` to try. The `tuneGrid` argument expects a data frame, which `expand.grid()` returns. We don't actually need `expand.grid()` for this example, but it will be a useful habit to develop when we move to methods with multiple tuning parameters.

```{r}
head(default_knn_mod$results, 5)
```

Since how we have a large number of results, display the entire `results` would create a lot of clutter. Instead, we can plot the tuning results by calling `plot()` on the object returned by `train()`.

```{r}
plot(default_knn_mod)
```

By default, `caret` utilizes the `lattice` graphics package to create these plots. Recently, additional support for `ggplot2` style graphics has been added for some plots.

```{r}
ggplot(default_knn_mod) + theme_bw()
```

Now that we are dealing with a tuning parameter, `train()` determines the best value of those considered, by default selecting the best (highest cross-validated) accuracy, and returning that value as `bestTune`.

```{r}
default_knn_mod$bestTune
```

```{r}
get_best_result = function(caret_fit) {
  best = which(rownames(caret_fit$results) == rownames(caret_fit$bestTune))
  best_result = caret_fit$results[best, ]
  rownames(best_result) = NULL
  best_result
}
```

Sometimes it will be useful to obtain the results for only that value. The above function does this automatically.

```{r}
get_best_result(default_knn_mod)
```

While we did fit a large number of models, the "best" model is stored in `finalModel`. After this model was determined to be the best via cross-validation, it is then fit to the entire training dataset.

```{r}
default_knn_mod$finalModel
```

With this model, we can again make predictions and obtain predicted probabilities.

```{r}
head(predict(default_knn_mod, newdata = default_tst, type = "prob"))
```

As an example of a multi-class response consider the following three models fit to the the `iris` data. Note that the first model is essentially "multinomial logistic regression," but you might notice it also has a tuning parameter now. (Spoiler: It's actually a neural network, so you'll need the `nnet` package.)

```{r}
iris_log_mod = train(
  Species ~ .,
  data = iris,
  method = "multinom",
  trControl = trainControl(method = "cv", number = 5),
  trace = FALSE
)
```

```{r}
iris_knn_mod = train(
  Species ~ .,
  data = iris,
  method = "knn",
  trControl = trainControl(method = "cv", number = 5),
  preProcess = c("center", "scale"),
  tuneGrid = expand.grid(k = seq(1, 21, by = 2))
)
```

```{r}
iris_qda_mod = train(
  Species ~ .,
  data = iris,
  method = "qda",
  trControl = trainControl(method = "cv", number = 5)
)
```

We can obtain predicted probabilities with these three models. Notice that they give the predicted probability for **each** class, using the same syntax for each model.

```{r}
head(predict(iris_log_mod, type = "prob"))
head(predict(iris_knn_mod, type = "prob"))
head(predict(iris_qda_mod, type = "prob"))
```


## Regression

To illustrate the use of `caret` for regression, we'll consider some simulated data.

```{r}
gen_some_data = function(n_obs = 50) {
  x1 = seq(0, 10, length.out = n_obs)
  x2 = runif(n = n_obs, min = 0, max = 2)
  x3 = sample(c("A", "B", "C"), size = n_obs, replace = TRUE)
  x4 = round(runif(n = n_obs, min = 0, max = 5), 1)
  x5 = round(runif(n = n_obs, min = 0, max = 5), 0)
  y = round(x1 ^ 2 + x2 ^ 2 + 2 * (x3 == "B") + rnorm(n = n_obs), 3)
  data.frame(y, x1, x2, x3, x4, x5)
}
```

We first simulate a train and test dataset.

```{r}
set.seed(42)
sim_trn = gen_some_data(n_obs = 500)
sim_tst = gen_some_data(n_obs = 5000)
```

Fitting `knn` works nearly identically to its use for classification. Really, the only difference here is that we have a numeric response, which `caret` understands to be a regression problem.

```{r}
sim_knn_mod = train(
  y ~ .,
  data = sim_trn,
  method = "knn",
  trControl = trainControl(method = "cv", number = 5),
  # preProcess = c("center", "scale"),
  tuneGrid = expand.grid(k = seq(1, 31, by = 2))
)
```

```{r}
sim_knn_mod$modelType
```

Notice that we've commented out the line to perform pre-processing. Can you figure out why?

```{r}
get_best_result(sim_knn_mod)
```

A few things to notice in the results. In addition to the usual RMSE, which is be used to determine the best model, we also have MAE, the [mean absolute error](https://en.wikipedia.org/wiki/Mean_absolute_error). We are also given standard deviations of both of these metrics.

```{r}
plot(sim_knn_mod)
```

The following plot adds error bars to RMSE estimate for each `k`.

```{r, echo = FALSE}
eps = 0.25
k = seq(1, 31, by = 2)
est = sim_knn_mod$results$RMSE
se = sim_knn_mod$results$RMSESD

plot(k, est, col = "dodgerblue", type = "b", ylim = c(3, 6.5), 
     pch = 20, lwd = 2, ylab = "RMSE (Cross-Validation)")
grid()
segments(k, est - se, k, est + se)
segments(k - eps, est - se, k + eps, est - se)
segments(k - eps, est + se, k + eps, est + se)
```

Sometimes, instead of simply picking the model with the best RMSE (or accuracy), we pick the simplest model within one standard error of the model with the best RMSE. Here then, we would consider `k = 11` instead of `k = 7` since there isn't a statistically significant difference. This is potentially a very good idea in practice. By picking a a simpler model, we are essentially at less risk of overfitting, especially since in practice, future data may be slightly different than the data that we are training on. If you're trying to win a Kaggle competition, this might not be as useful, since often the test and train data come from the exact same source.

- TODO: additional details about 1-SE rule.

```{r}
calc_rmse = function(actual, predicted) {
  sqrt(mean((actual - predicted) ^ 2))
}
```

Since we simulated this data, we have a rather large test dataset. This allows us to compare our cross-validation error estimate, to an estimate using (an impractically large) test set.

```{r}
get_best_result(sim_knn_mod)$RMSE
```

```{r}
calc_rmse(actual = sim_tst$y,
          predicted = predict(sim_knn_mod, sim_tst))
```

Here we see that the cross-validated RMSE is a bit of an overestimate, but still rather close to the test error. The real question is, are either of these any good? Is this model predicting well? No! Notice that we simulated this data with an error standard deviation of 1!


### Methods

Now that `caret` has given us a pipeline for a predictive analysis, we can very quickly and easily test new methods. For example, in an upcoming chapter we will discuss boosted tree models, but now that we understand how to use `caret`, in order to *use* a boosted tree model, we simply need to know the "method" to do so, which in this case is `gbm`. Beyond knowing that the method exists, we just need to know its tuning parameters, in this case, there are four. We actually could get away with knowing nothing about them, and simply specify a `tuneLength`, then `caret` would automatically try some reasonable values.

Instead, we could [read the `caret` documentation](https://topepo.github.io/caret/train-models-by-tag.html) to find the tuning parameters, as well as the required packages. For now, we'll simply use the following tuning grid. In later chapters, we'll discuss how these effect the model.

```{r}
gbm_grid = expand.grid(interaction.depth = c(1, 2, 3),
                       n.trees = (1:30) * 100,
                       shrinkage = c(0.1, 0.3),
                       n.minobsinnode = 20)
head(gbm_grid)
```

```{r message = FALSE, warning = FALSE}
set.seed(42)
sim_gbm_mod = train(
  y ~ .,
  data = sim_trn,
  trControl = trainControl(method = "cv", number = 5),
  method = "gbm",
  tuneGrid = gbm_grid,
  verbose = FALSE
)
```

We added `verbose = FALSE` to the `train()` call to suppress some of the intermediate output of the `gbm` fitting procedure.

How this training is happening is a bit of a mystery to us right now. What is this method? How does it deal with the factor variable as a predictor? We'll answer these questions later, for now, we do know how to evaluate how well the method is working.

```{r}
knitr::kable(head(sim_gbm_mod$results), digits = 3)
```

```{r}
plot(sim_gbm_mod)
```

```{r}
sim_gbm_mod$bestTune
```

Here we obtain the set of tuning parameters that performed best. Based on the above plot, do you think we considered enough possible tuning parameters?

```{r}
get_best_result(sim_gbm_mod)
```


```{r}
calc_rmse(actual = sim_tst$y,
          predicted = predict(sim_gbm_mod, sim_tst))
```

Again, the cross-validated result is overestimating the error a bit. Also, this model is a big improvement over the `knn` model, but we can still do better.

```{r}
sim_lm_mod = train(
  y ~ poly(x1, 2) + poly(x2, 2) + x3,
  data = sim_trn,
  method = "lm",
  trControl = trainControl(method = "cv", number = 5)
)
```

```{r}
sim_lm_mod$finalModel
```

Here we fit a good old linear model, except, we specify a very specific formula.

```{r}
sim_lm_mod$results$RMSE
```

```{r}
calc_rmse(actual = sim_tst$y,
          predicted = predict(sim_lm_mod, sim_tst))
```

This model dominates the previous two. The `gbm` model does still have a big advantage. The `lm` model needed the correct form of the model, whereas `gbm` nearly learned it automatically!

This question of *which* variables should be included is where we will turn our focus next. We'll consider both what variables are useful for prediction, and learn tools to asses how useful they are.


## External Links

- [The `caret` Package](http://topepo.github.io/caret/index.html) - Reference documentation for the `caret` package in `bookdown` format.
- [`caret` Model List](http://topepo.github.io/caret/available-models.html) - List of available models in `caret`.
- [`caret` Model List, By Tag](https://topepo.github.io/caret/train-models-by-tag.html) - Gives information on tuning parameters and necessary packages.
- [Applied Predictive Modeling](http://www.springer.com/us/book/9781461468486) - Book from the author of the `caret` package, Max Kuhn, as well as Kjell Johnson. Further discussion on use of statistical learning methods in practice.


## `rmarkdown`

The `rmarkdown` file for this chapter can be found [**here**](21-caret.Rmd). The file was created using `R` version `r paste0(version$major, "." ,version$minor)`. The following packages (and their dependencies) were loaded when knitting this file:

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