---
title: "Introduction to mikropml"
author: "Zena Lapp"
output: rmarkdown::html_vignette
bibliography: paper.bib
vignette: >
  %\VignetteIndexEntry{Introduction to mikropml}
  %\VignetteEngine{knitr::rmarkdown}
  %\VignetteEncoding{UTF-8}
---

```{r, include = FALSE}
knitr::opts_chunk$set(
  collapse = TRUE,
  comment = "#>"
)
```

The goal of `mikropml` is to make supervised machine learning (ML) easy for you
to run while implementing good practices for machine learning pipelines. All you
need to run the ML pipeline is one function: `run_ml()`. We've selected sensible default
arguments related to good practices [@topcuoglu_framework_2020;
@tang_democratizing_2020], but we allow you to change those arguments to tailor
`run_ml()` to the needs of your data.

This document takes you through all of the `run_ml()` inputs, both required and
optional, as well as the outputs.

In summary, you provide:

- A dataset with an outcome column and feature columns (rows are samples; unfortunately we do not support multi-label classification)
- Model choice (i.e. method)

And the function outputs:

- The trained model
- Model performance metrics
- (Optional) feature importance metrics

# It's running so slow!

Since I assume a lot of you won't read this entire vignette, I'm going to say
this at the beginning. If the `run_ml()` function is running super slow, you
should consider parallelizing. See `vignette("parallel")` for examples.

# Understanding the inputs

## The input data

The input data to `run_ml()` is a dataframe where each row is a sample or
observation. One column (assumed to be the first) is the outcome of interest,
and all of the other columns are the features. We package `otu_mini_bin` as a
small example dataset with `mikropml`.

```{r}
#install.packages("devtools")
#devtools::install_github("SchlossLab/mikropml")
library(mikropml)
head(otu_mini_bin)
```

Here, `dx` is the outcome column (normal or cancer), and there are 10 features
(`Otu00001` through `Otu00010`). Because there are only 2 outcomes, we will be
performing binary classification in the majority of the examples below. At the
bottom, we will also briefly provide examples of multi-class and continuous
outcomes. As you'll see, you run them in the same way as for binary
classification!

The feature columns are the amount of each [Operational Taxonomic Unit
(OTU)](https://en.wikipedia.org/wiki/Operational_taxonomic_unit) in microbiome
samples from patients with cancer and without cancer. The goal is to predict
`dx`, which stands for diagnosis. This diagnosis can be cancer or not based on
an individual's microbiome. No need to understand exactly what that means, but
if you're interested you can read more about it from the original paper
[@topcuoglu_framework_2020].

For real machine learning applications you'll need to use more features, but for
the purposes of this vignette we'll stick with this example dataset so
everything runs faster.

## The methods we support

All of the methods we use are supported by a great ML wrapper package
[`caret`](https://topepo.github.io/caret/), which we use to train our machine
learning models.

The methods we have tested (and their backend packages) are:

- Logistic/multiclass/linear regression (`"glmnet"`)
- Random forest (`"rf"`)
- Decision tree (`"rpart2"`)
- Support vector machine with a radial basis kernel (`"svmRadial"`)
- xgboost (`"xgbTree"`)

For documentation on these methods, as well as many others, you can look at the
[available models](https://topepo.github.io/caret/available-models.html) (or see
[here](https://topepo.github.io/caret/train-models-by-tag.html) for a list by
tag). While we have not vetted the other models used by `caret`, our function is
general enough that others might work. While we can't promise that we can help
with other models, feel free to [open an issue on
GitHub](https://github.com/SchlossLab/mikropml/issues) if you have questions
about other models and we _might_ be able to help.

We will first focus on `glmnet`, which is our default implementation of
L2-regularized logistic regression. Then we will cover a few other examples
towards the end.

# Before running ML

Before you execute `run_ml()`, you should consider preprocessing your data,
either on your own or with the `preprocess_data()` function. You can learn more
about this in the preprocessing vignette: `vignette("preprocess")`.

# The simplest way to `run_ml()`

As mentioned above, the minimal input is your dataset (`dataset`) and the
machine learning model you want to use (`method`).

You may also want to provide:

- The outcome column name. By default `run_ml()` will pick the first column, but it's best practice to specify the column name explicitly.
- A seed so that the results will be reproducible, and so that you get the same results as those you see here (i.e have the same train/test split).

Say we want to use logistic regression, then the method we will use is `glmnet`.
To do so, run the ML pipeline with:

```{r, eval = FALSE}
results <- run_ml(otu_mini_bin,
                  'glmnet',
                  outcome_colname = 'dx',
                  seed = 2019)
```
```{r, echo = FALSE}
# reduce vignette runtime by using precomputed results
results <- otu_mini_bin_results_glmnet
```

You'll notice a few things:

1. It takes a little while to run. This is because of some of the parameters we use.
1. There is a message stating that 'dx' is being used as the outcome column. This is what we want, but it's a nice sanity check!
1. There was a warning. Don't worry about this warning right now - it just means that some of the hyperparameters aren't a good fit - but if you're interested in learning more, see `vignette("tuning")`.

Now, let's dig into the output a bit.
The results is a list of 4 things:

```{r}
names(results)
```

`trained_model` is the trained model from `caret`.
There is a bunch of info in this that we won't get into,
because you can learn more from the `caret::train()` documentation.

```{r}
names(results$trained_model)
```

`test_data` is the partition of the dataset that was used for testing. In
machine learning, it's always important to have a held-out test dataset that is
not used in the training stage. In this pipeline we do that using `run_ml()`
where we split your data into training and testing sets. The training data are
used to build the model (e.g. tune hyperparameters, learn the data) and the test
data are used to evaluate how well the model performs.

```{r}
head(results$test_data)
```

`performance` is a dataframe of (mainly) performance metrics (1 column for
cross-validation performance metric, several for test performance metrics, and 2
columns at the end with ML method and seed):

```{r}
results$performance
```

When using logistic regression for binary classification, area under the
receiver-operator characteristic curve (AUC) is a useful metric to evaluate
model performance. Because of that, it's the default that we use for `mikropml`.
However, it is crucial to evaluate your model performance using multiple
metrics. Below you can find more information about other performance metrics and
how to use them in our package.

`cv_metric_AUC` is the AUC for the cross-validation folds for the training data.
This gives us a sense of how well the model performs on the training data.

Most of the other columns are performance metrics for the test data — the data
that wasn't used to build the model. Here, you can see that the AUC for the test
data is not much above 0.5, suggesting that this model does not predict much
better than chance, and that the model is overfit because the cross-validation
AUC (`cv_metric_AUC`, measured during training) is much higher than the testing
AUC. This isn't too surprising since we're using so few features with this
example dataset, so don't be discouraged. The default option also provides a
number of other performance metrics that you might be interested in, including
area under the precision-recall curve (prAUC).

The last columns of `results$performance` are the method and seed (if you set
one) to help with combining results from multiple runs (see
`vignette("parallel")`).

`feature_importance` has information about feature importance values if
`find_feature_importance = TRUE` (the default is `FALSE`). Since we used the
defaults, there's nothing here:

```{r}
results$feature_importance
```

# Customizing parameters

There are a few arguments that allow you to change how you execute `run_ml()`.
We've chosen reasonable defaults for you, but we encourage you to change these
if you think something else would be better for your data.

## Changing `kfold`, `cv_times`, and `training_frac`

- `kfold`: The number of folds to run for cross-validation (default: 5).
- `cv_times`: The number of times to run repeated cross-validation (default: 100).
- `training_frac`: The fraction of data for the training set (default: 0.8). The rest of the data is used for testing.

Here's an example where we change some of the default parameters:

```{r}
results_custom <- run_ml(otu_mini_bin,
                         'glmnet',
                         kfold = 2,
                         cv_times = 5,
                         training_frac = 0.5,
                         seed = 2019)
```

You might have noticed that this one ran faster — that's because we reduced `kfold` and `cv_times`.
This is okay for testing things out and may even be necessary for smaller datasets. 
But in general it may be better to have larger numbers for these parameters; 
we think the defaults are a good starting point [@topcuoglu_framework_2020].

### Custom training indices

When `training_frac` is a fraction between 0 and 1, a random sample of
observations in the dataset are chosen for the training set to satisfy the
`training_frac`. However, in some cases you might wish to control exactly which
observations are in the training set. You can instead assign `training_frac` a
vector of indices that correspond to which rows of the dataset should go in the
training set (all remaining sequences will go in the testing set).

```{r custom_train_indices, warning=FALSE}
n_obs <- otu_mini_bin %>% nrow()
training_size <- 0.8 * n_obs
training_rows <- sample(n_obs, training_size)
results_custom_train <- run_ml(otu_mini_bin,
                               'glmnet',
                               kfold = 2,
                               cv_times = 5,
                               training_frac = training_rows,
                               seed = 2019
                               )
```


## Changing the performance metric

There are two arguments that allow you to change what performance metric to use
for model evaluation, and what performance metrics to calculate using the test
data.

`perf_metric_function` is the function used to calculate the performance
metrics.

The default for classification is `caret::multiClassSummary()` and the default
for regression is `caret::defaultSummary()`. We'd suggest not changing this
unless you really know what you're doing.

`perf_metric_name` is the column name from the output of `perf_metric_function`.
We chose reasonable defaults (AUC for binary, logLoss for multiclass, and RMSE
for continuous), but the default functions calculate a bunch of different
performance metrics, so you can choose a different one if you'd like.

The default performance metrics available for classification are:

```{r, echo=FALSE}
# TODO: can we get these programmatically somehow instead of hard-coding them?
c("logLoss", "AUC", "prAUC", "Accuracy", "Kappa", "Mean_F1", "Mean_Sensitivity", "Mean_Specificity", "Mean_Pos_Pred_Value", "Mean_Neg_Pred_Value", "Mean_Precision", "Mean_Recall", "Mean_Detection_Rate", "Mean_Balanced_Accuracy")
```

The default performance metrics available for regression are:

```{r, echo=FALSE}
c("RMSE", "Rsquared", "MAE")
```

Here's an example using prAUC instead of AUC:

```{r}
results_pr <- run_ml(otu_mini_bin, 
                     'glmnet', 
                     cv_times = 5, 
                     perf_metric_name = 'prAUC', 
                     seed = 2019)
```

You'll see that the cross-validation metric is prAUC, instead of the default AUC:

```{r}
results_pr$performance
```

## Using groups

The optional `groups` is a vector of groups to keep together when splitting the
data into train and test sets and for cross-validation. Sometimes it's
important to split up the data based on a grouping instead of just randomly.
This allows you to control for similarities within groups that you don't want to
skew your predictions (i.e. batch effects). For example, with biological data
you may have samples collected from multiple hospitals, and you might like to
keep observations from the same hospital in the same partition.

Here's an example where we split the data into train/test sets based on groups:

```{r custom_groups, warning=FALSE}
# make random groups
set.seed(2019)
grps <- sample(LETTERS[1:8], nrow(otu_mini_bin), replace=TRUE)
results_grp <- run_ml(otu_mini_bin, 
                      'glmnet', 
                      cv_times = 2, 
                      training_frac = 0.8, 
                      groups = grps, 
                      seed = 2019)
```

The one difference here is `run_ml()` will report how much of the data is in the
training set if you run the above code chunk. This can be a little finicky
depending on how many samples and groups you have. This is because it won't be
exactly what you specify with `training_frac`, since you have to include all of
one group in either the training set _or_ the test set.

### Controling how groups are assigned to partitions

When you use the `groups` parameter as above, by default `run_ml()` will assume
that you want all of the observations from each group to be placed in the same
partition of the train/test split. This makes sense when you want to use groups
to control for batch effects. However, in some cases you might prefer to control
exactly which groups end up in which partition, and you might even be okay with
some observations from the same group being assigned to different partitions.

For example, say you want groups A and B to be used for training, C and D for
testing, and you don't have a preference for what happens to the other groups.
You can give the `group_partitions` parameter a named list to specify which
groups should go in the training set and which should go in the testing set.

```{r group_partitions, warning=FALSE}
results_grp_part <- run_ml(otu_mini_bin, 
                      'glmnet', 
                      cv_times = 2, 
                      training_frac = 0.8, 
                      groups = grps, 
                      group_partitions = list(train = c('A', 'B'),
                                              test = c('C', 'D')
                                              ),
                      seed = 2019)
```

In the above case, all observations from A & B will be used for training, all
from C & D will be used for testing, and the remaining groups will be randomly
assigned to one or the other to satisfy the `training_frac` as closely as
possible.

In another scenario, maybe you want only groups A through F to be used for
training, but you also want to allow other observations not selected for
training from A through F to be used for testing:

```{r only_group_A_train, warning = FALSE}
results_grp_trainA <- run_ml(otu_mini_bin, 
                      'glmnet', 
                      cv_times = 2, 
                      kfold = 2,
                      training_frac = 0.5, 
                      groups = grps, 
                      group_partitions = list(train = c("A", "B", "C", "D", "E", "F"),
                                              test = c("A", "B", "C", "D", "E", "F", "G", "H")
                                              ),
                      seed = 2019)
```

If you need even more control than this, take a look at 
[setting custom training indices](#custom-training-indices). 
You might also prefer to  provide your own train control scheme with the
`cross_val` parameter in `run_ml()`.

# Finding feature importance

To find which features are contributing to predictive power, you can use
`find_feature_importance = TRUE`. How we use permutation importance to determine
feature importance is described in [@topcuoglu_framework_2020]. Briefly, it
permutes each of the features individually (or correlated ones together) and
evaluates how much the performance metric decreases. The more performance
decreases when the feature is randomly shuffled, the more important that feature
is. The default is `FALSE` because it takes a while to run and is only useful if
you want to know what features are important in predicting your outcome.

Let's look at some feature importance results:

```{r, eval = FALSE}
results_imp <- run_ml(otu_mini_bin,
  "rf",
  outcome_colname = "dx",
  find_feature_importance = TRUE,
  seed = 2019
)
```
```{r, echo = FALSE}
results_imp <- otu_mini_bin_results_rf
```

Now, we can check out the feature importances:

```{r}
results_imp$feature_importance
```

There are several columns:

1. `perf_metric`: The performance value of the permuted feature.
1. `perf_metric_diff`: The difference between the performance for the actual and permuted data (i.e. test performance minus permuted performance). Features with a larger `perf_metric_diff` are more important.
1. `pvalue`: the probability of obtaining the actual performance value under the null hypothesis.
1. `names`: The feature that was permuted.
1. `method`: The ML method used.
1. `perf_metric_name`: The peformance metric used.
1. `seed`: The seed (if set).

As you can see here, the differences are negligible (close to zero), which makes
sense since our model isn't great. If you're interested in feature importance,
it's especially useful to run multiple different train/test splits, as shown in
our 
[example snakemake workflow](https://github.com/SchlossLab/mikropml-snakemake-workflow/).

You can also choose to permute correlated features together using `corr_thresh`
(default: 1). Any features that are above the correlation threshold are permuted
together; i.e. perfectly correlated features are permuted together when using
the default value.

```{r}
results_imp_corr <- run_ml(otu_mini_bin,
                           'glmnet',
                           cv_times = 5,
                           find_feature_importance = TRUE,
                           corr_thresh = 0.2,
                           seed = 2019)
results_imp_corr$feature_importance
```

You can see which features were permuted together in the `names` column. Here
all 3 features were permuted together (which doesn't really make sense, but it's
just an example).

If you previously executed `run_ml()` without feature importance but now wish
to find feature importance after the fact, see the example code in the
`get_feature_importance()` documentation.

`get_feature_importance()` can show a live progress bar, see
`vignette("parallel")` for examples.

# Tuning hyperparameters (using the `hyperparameter` argument)

This is important, so we have a whole vignette about them. The bottom line is we
provide default hyperparameters that you can start with, but it's important to
tune your hyperparameters. For more information about what the default
hyperparameters are, and how to tune hyperparameters, see `vignette("tuning")`.

# Other models

Here are examples of how to train and evaluate other models.
The output for all of them is very similar, so we won't go into those details.

## Random forest

```{r, eval = FALSE}
results_rf <- run_ml(otu_mini_bin,
                     'rf',
                     cv_times = 5,
                     seed = 2019)
```

You can also change the number of trees to use for random forest 
(`ntree`; default: 1000). 
This can't be tuned using `rf` package implementation of random forest. 
Please refer to `caret` documentation if you are interested in 
other packages with random forest implementations.

```{r, eval = FALSE}
results_rf_nt <- run_ml(otu_mini_bin,
                        'rf',
                        cv_times = 5,
                        ntree = 10,
                        seed = 2019)
```

## Decision tree

```{r, eval = FALSE}
results_dt <- run_ml(otu_mini_bin,
                     'rpart2',
                     cv_times = 5,
                     seed = 2019)
```

## SVM

```{r, eval = FALSE}
results_svm <- run_ml(otu_mini_bin,
                      'svmRadial',
                      cv_times = 5,
                      seed = 2019)
```

If you get a message "maximum number of iterations reached", see [this issue](https://github.com/topepo/caret/issues/425) in caret.

# Other data

## Multiclass data

We provide `otu_mini_multi` with a multiclass outcome (three or more outcomes): 

```{r}
otu_mini_multi %>% dplyr::pull('dx') %>% unique()
```


Here's an example of running multiclass data:

```{r, eval = FALSE}
results_multi <- run_ml(otu_mini_multi,
                        outcome_colname = "dx",
                        seed = 2019
)
```
```{r, echo = FALSE}
results_multi <- otu_mini_multi_results_glmnet
```

The performance metrics are slightly different,
but the format of everything else is the same:

```{r}
results_multi$performance
```

## Continuous data

And here's an example for running continuous data, 
where the outcome column is numerical:

```{r, eval = FALSE}
results_cont <- run_ml(otu_mini_bin[, 2:11],
                       'glmnet',
                       outcome_colname = 'Otu00001',
                       seed = 2019)
```
```{r, echo = FALSE}
results_cont <- otu_mini_cont_results_glmnet
```

Again, the performance metrics are slightly different,
but the format of the rest is the same:

```{r}
results_cont$performance
```

# References
