---
title: "*ropls*: PCA, PLS(-DA) and OPLS(-DA) for multivariate analysis and
feature selection of omics data"
author: "Etienne A. Thevenot"
date: "`r doc_date()`"
package: "`r pkg_ver('ropls')`"

vignette: >
  %\VignetteIndexEntry{Vignette Title}
  %\VignetteEngine{knitr::rmarkdown}
  %\VignetteEncoding{UTF-8}
  %\VignetteKeywords{Regression, Classification, PrincipalComponent, Transcriptomics,
    Proteomics, Metabolomics, Lipidomics, MassSpectrometry}
bibliography: "ropls-vignette.bib"
output:
  BiocStyle::html_document
---

```{r global_options, include=FALSE}
knitr::opts_chunk$set(fig.width = 6,
                      fig.height = 6,
                      fig.path = 'figures/')
```

# The *ropls* package

The [ropls](http://bioconductor.org/packages/release/bioc/html/ropls.html) R
package implements the **PCA**, **PLS(-DA)** and **OPLS(-DA)** approaches with
the original, **NIPALS**-based, versions of the algorithms
[@Wold2001, @Trygg2002]. It includes the **R2** and **Q2** quality metrics
[@Eriksson2001, @Tenenhaus1998], the permutation **diagnostics**
[@Szymanska2012], the computation of the **VIP** values [@Wold2001], the score
and orthogonal distances to detect **outliers** [@Hubert2005], as well as many
**graphics** (scores, loadings, predictions, diagnostics, outliers, etc).

The functionalities from
[ropls](http://bioconductor.org/packages/release/bioc/html/ropls.html) can also
be accessed via a graphical user interface in the **Multivariate** module from
the [Workflow4Metabolomics.org](http://workflow4metabolomics.org) online
resource for computational metabolomics, which provides a user-friendly,
**Galaxy**-based environment for data pre-processing, statistical analysis, and
annotation [@Giacomoni2015].

# Context

## Orthogonal Partial Least-Squares

**Partial Least-Squares (PLS)**, which is a latent variable regression method
based on covariance between the predictors and the response, has been shown to
efficiently handle datasets with multi-collinear predictors, as in the case of
spectrometry measurements [@Wold2001]. More recently, @Trygg2002 introduced the
**Orthogonal Partial Least-Squares (OPLS)** algorithm to model separately the
variations of the predictors correlated and orthogonal to the response.

OPLS has a similar predictive capacity compared to PLS and improves the
**interpretation** of the predictive components and of the systematic variation
[@Pinto2012a]. In particular, OPLS modeling of single responses only requires
one predictive component.

**Diagnostics** such as the **Q2Y** metrics and permutation testing are of high
importance to **avoid overfitting** and assess the statistical significance of
the model. The **Variable Importance in Projection (VIP)**, which reflects both
the loading weights for each component and the variability of the response
explained by this component [@Pinto2012a; @Mehmood2012], can be used for
feature selection [@Trygg2002; @Pinto2012a].

## OPLS software

OPLS is available in the **SIMCA-P** commercial software (Umetrics, Umea,
Sweden; @Eriksson2001). In addition, the kernel-based version of OPLS
[@Bylesjo2008a] is available in the open-source R statistical environment
[@RCoreTeam2016], and a single implementation of the linear algorithm in R has
been described recently [@Gaude2013].

# The *sacurine* metabolomics dataset

## Study objective

The objective was to study the influence of **age**, **body mass index (bmi)**,
and **gender** on metabolite concentrations in **urine**, by analysing 183
samples from a **cohort** of adults with liquid chromatography coupled to
high-resolution mass spectrometry (**LC-HRMS**; @Thevenot2015).

## Pre-processing and annotation

Urine samples were analyzed by using an LTQ Orbitrap in the negative ionization
mode. A total of **109 metabolites** were identified or annotated at the MSI level 1
or 2. After retention time alignment with XCMS, peaks were integrated with Quan
Browser. Signal drift and batch effect were corrected, and each urine profile
was normalized to the osmolality of the sample. Finally, the data were log10
transformed [@Thevenot2015].

## Covariates

The volunteers' **age**, **body mass index (bmi)**, and **gender** were recorded.

# Hands-on

## Loading

We first load the
[ropls](http://bioconductor.org/packages/release/bioc/html/ropls.html) package:

```{r load, message = FALSE}
library(ropls)
```

We then load the **sacurine** dataset which contains:

1. The **dataMatrix** matrix of numeric type containing the intensity profiles
   (log10 transformed),

2. The **sampleMetadata** data frame containg sample metadata,

3. The **variableMetadata** data frame containg variable metadata

```{r sacurine}
data(sacurine)
names(sacurine)
```

We attach **sacurine** to the search path and display a summary of the content
of the **dataMatrix**, **sampleMetadata** and **variableMetadata** with the
`strF` *F*unction of the
[ropls](http://bioconductor.org/packages/release/bioc/html/ropls.html) package
(see also `str`):

```{r attach_code, message = FALSE}
attach(sacurine)
```
```{r strF}
strF(dataMatrix)
strF(sampleMetadata)
strF(variableMetadata)
```

We may also have a look at the data matrix with the `imageF` function:

```{r imageF}
imageF(dataMatrix)
```

## Principal Component Analysis (PCA)

We perform a **PCA **on the **dataMatrix** matrix (samples as rows, variables as
columns), with the `opls` method:

```{r pca_code, eval = FALSE}
sacurine.pca <- opls(dataMatrix)
```

A **summary** of the model (8 components were selected) is printed:

```{r pca_result, echo = FALSE}
sacurine.pca <- opls(dataMatrix, fig.pdfC = NULL)
```

In addition the default **summary** figure is displayed:

```{r pca_figure, echo = FALSE, fig.show = 'hold'}
plot(sacurine.pca)
```
**Figure 1: PCA summary plot.** **Top left** `overview`: the **scree** plot (i.e.,
inertia barplot) suggests that 3 components may be sufficient to capture most of
the inertia; **Top right** `outlier`: this graphics shows the distances within
and orthogonal to the projection plane [@Hubert2005]: the name of the samples
with a high value for at least one of the distances are indicated; **Bottom
left** `x-score`: the variance along each axis equals the variance captured by
each component: it therefore depends on the total variance of the dataMatrix *X*
and of the percentage of this variance captured by the component (indicated in
the labels); it decreases when going from one component to a component with
higher indice; **Bottom right** `x-loading`: the 3 variables with most extreme
values (positive and negative) for each loading are black colored and labeled.

Note:

1. Since **dataMatrix** does not contain missing value, the singular value
   decomposition was used by default; NIPALS can be selected with the
   `algoC` argument specifying the *algo*rithm (*C*haracter),

2. The `predI = NA` default number of *pred*ictive components (*I*nteger) for
   PCA means that components (up to 10) will be computed until the cumulative
   variance exceeds 50%. If the 50% have not been reached at the 10th component,
   a warning message will be issued (you can still compute the following
   components by specifying the `predI` value).

Let us see if we notice any partition according to gender, by labeling/coloring
the samples according to *gender* (`parAsColFcVn`) and drawing the Mahalanobis
ellipses for the male and female subgroups (`parEllipseL`).

```{r pca-col}
genderFc <- sampleMetadata[, "gender"]
plot(sacurine.pca,
     typeVc = "x-score",
     parAsColFcVn = genderFc)
```
**Figure 2: PCA score plot colored according to gender.**

Note:

1. The plotting *par*ameter to be used *As* *Col*ors
(*F*actor of *c*haracter type or *V*ector of
*n*umeric type) has a length equal to the number of rows of the
**dataMatrix** (ie of samples) and that this qualitative or quantitative
variable is converted into colors (by using an internal palette or color scale,
respectively). We could have visualized the *age* of the individuals by
specifying `parAsColFcVn = sampleMetadata[, "age"]`.

2. The displayed components can be specified with `parCompVi` (plotting
*par*ameter specifying the *Comp*onents: *V*ector of 2 *i*ntegers)

3. The labels and the color palette can be personalized with the `parLabVc` and `parPaletteVc` parameters, respectively:
```{r pca-col-personalized}
plot(sacurine.pca,
     typeVc = "x-score",
     parAsColFcVn = genderFc,
     parLabVc = as.character(sampleMetadata[, "age"]),
     parPaletteVc = c("green4", "magenta"))
```


## Partial least-squares: PLS and PLS-DA

For **PLS (and OPLS)**, the **Y** response(s) must be provided to the `opls`
method. **Y** can be either a numeric vector (respectively matrix) for single
(respectively multiple) **(O)PLS regression**, or a character factor for
**(O)PLS-DA classification** as in the following example with the *gender*
qualitative response:

```{r plsda}
sacurine.plsda <- opls(dataMatrix, genderFc)
```
**Figure 3: PLS-DA model of the gender response.** **Top left**:
`significance` diagnostic: the **R2Y** and **Q2Y** of the model are compared
with the corresponding values obtained after random permutation of the *y*
response; **Top right**: `inertia` barplot: the graphic here suggests that 3
orthogonal components may be sufficient to capture most of the inertia; **Bottom
left**: `outlier` diagnostics; **Bottom right**: `x-score` plot: the number of
components and the cumulative **R2X**, **R2Y** and **Q2Y** are indicated below the
plot.

Note:

1. When set to NA (as in the default), **the number of components** `predI` is
   determined automatically as follows [@Eriksson2001]: A new component *h* is
   added to the model if:

  + $R2Y_h \geq 0.01$, i.e., the percentage of **Y** dispersion (i.e., sum of
    squares) explained by component *h* is more than 1 percent, and

  + $Q2Y_h=1-PRESS_h/RSS_{h-1} \geq 0$ for PLS (or 5% when the number of
    samples is less than 100) or 1% for OPLS: $Q2Y_h \geq 0$ means that the
    predicted residual sum of squares ($PRESS_h$) of the model including the new
    component *h* estimated by 7-fold cross-validation is less than the residual
    sum of squares ($RSS_{h-1}$) of the model with the previous components only
    (with $RSS_0$ being the sum of squared **Y** values).

2. The **predictive performance** of the full model is assessed by the
cumulative **Q2Y** metric: $Q2Y=1-\prod\limits_{h=1}^r (1-Q2Y_h)$. We have $Q2Y
\in [0,1]$, and the higher the **Q2Y**, the better the performance. Models
trained on datasets with a larger number of features compared with the number of
samples can be prone to **overfitting**: in that case, high **Q2Y** values are
obtained by chance only. To estimate the significance of **Q2Y** (and **R2Y**)
for single response models, permutation testing [@Szymanska2012] can be used:
models are built after random permutation of the **Y** values, and $Q2Y_{perm}$
are computed. The *p*-value is equal to the proportion of $Q2Y_{perm}$ above
$Q2Y$ (the **default number of permutations is 20** as a compromise between quality
control and computation speed; it can be increased with the `permI` parameter,
e.g. to 1,000, to assess if the model is significant at the 0.05 level),

3. The **NIPALS** algorithm is used for PLS (and OPLS); *dataMatrix* matrices
with (a moderate amount of) missing values can thus be analysed.

We see that our model with 3 predictive (*pre*) components has significant and
quite high R2Y and Q2Y values.

## Orthogonal partial least squares: OPLS and OPLS-DA

To perform **OPLS(-DA)**, we set `orthoI` (number of components which are
*ortho*gonal; *I*nteger) to either a specific number of orthogonal components,
or to NA. Let us build an OPLS-DA model of the *gender* response.

```{r oplsda}
sacurine.oplsda <- opls(dataMatrix, genderFc,
                        predI = 1, orthoI = NA)
```
**Figure 4: OPLS-DA model of the gender response.**

Note:

1. For OPLS modeling of a single response, the number of predictive component is
1,

2. In the (`x-score` plot), the predictive component is displayed as abscissa
and the (selected; default = 1) orthogonal component as ordinate.


Let us assess the **predictive performance** of our model. We first train the
model on a subset of the samples (here we use the `odd` subset value which
splits the data set into two halves with similar proportions of samples for each
class; alternatively, we could have used a specific subset of indices for
training):
```{r oplsda_subset, warning=FALSE}
sacurine.oplsda <- opls(dataMatrix, genderFc,
                        predI = 1, orthoI = NA,
                        subset = "odd")
```

We first check the predictions on the **training** subset:

```{r train}
trainVi <- getSubsetVi(sacurine.oplsda)
table(genderFc[trainVi], fitted(sacurine.oplsda))
```

We then compute the performances on the **test** subset:

```{r test}
table(genderFc[-trainVi],
      predict(sacurine.oplsda, dataMatrix[-trainVi, ]))
```

As expected, the predictions on the test subset are (slightly) lower. The
classifier however still achieves 91% of correct predictions.

## Comments

### Overfitting

**Overfitting** (i.e., building a model with good performances on the training
set but poor performances on a new test set) is a major caveat of machine
learning techniques applied to data sets with more variables than samples. A
simple simulation of a random **X** data set and a **y** response shows that
perfect PLS-DA classification can be achieved as soon as the number of variables
exceeds the number of samples, as detailed in the example below, adapted from
@Wehrens2011:

```{r overfit, echo = FALSE}
set.seed(123)
obsI <- 20
featVi <- c(2, 20, 200)
featMaxI <- max(featVi)
xRandMN <- matrix(runif(obsI * featMaxI), nrow = obsI)
yRandVn <- sample(c(rep(0, obsI / 2), rep(1, obsI / 2)))

layout(matrix(1:4, nrow = 2, byrow = TRUE))
for (featI in featVi) {
  randPlsi <- opls(xRandMN[, 1:featI], yRandVn,
                   predI = 2,
                   permI = ifelse(featI == featMaxI, 100, 0),
                   fig.pdfC = NULL,
                   info.txtC = NULL)
  plot(randPlsi, typeVc = "x-score",
       parCexN = 1.3, parTitleL = FALSE,
       parCexMetricN = 0.5)
  mtext(featI/obsI, font = 2, line = 2)
  if (featI == featMaxI)
    plot(randPlsi,
         typeVc = "permutation",
         parCexN = 1.3)
}
mtext(" obs./feat. ratio:",
      adj = 0, at = 0, font = 2,
      line = -2, outer = TRUE)
```
**Figure 5: Risk of PLS overfitting.** In the simulation above, a **random
matrix X** of 20 observations x 200 features was generated by sampling from the
uniform distribution $U(0, 1)$. A **random y** response was obtained by sampling
(without replacement) from a vector of 10 zeros and 10 ones. **Top left**, **top
right**, and **bottom left**: the X-**score plots** of the PLS modeling of **y**
by the (sub)matrix of **X** restricted to the first 2, 20, or 200 features, are
displayed (i.e., the observation/feature ratios are 0.1, 1, and 10,
respectively). Despite the good separation obtained on the bottom left score
plot, we see that the **Q2Y** estimation of predictive performance is low
(negative); **Bottom right**: a significant proportion of the models (in fact
here all models) trained after random permutations of the labels have a higher
**Q2Y** value than the model trained with the true labels, confirming that PLS
cannot specifically model the **y** response with the **X** predictors, as
expected.

This simple simulation illustrates that PLS overfit can occur, in particular
when the number of features exceeds the number of observations. **It is
therefore essential to check that the $Q2Y$ value of the model is significant by
random permutation of the labels**.

### VIP from OPLS models

The classical **VIP** metric is not useful for OPLS modeling of a single response
since [@Galindo-Prieto2014, @Thevenot2015]:
1. **VIP** values remain identical whatever the number of orthogonal components
selected,
2. **VIP** values are univariate (i.e., they do not provide information about
interactions between variables). In fact, when features are standardized, we
can demonstrate a mathematical relationship between VIP and *p*-values from a
Pearson correlation test [@Thevenot2015], as illustrated by the figure below:

```{r vip}
ageVn <- sampleMetadata[, "age"]

pvaVn <- apply(dataMatrix, 2,
               function(feaVn) cor.test(ageVn, feaVn)[["p.value"]])

vipVn <- getVipVn(opls(dataMatrix, ageVn,
                       predI = 1, orthoI = NA,
                       fig.pdfC = NULL))

quantVn <- qnorm(1 - pvaVn / 2)
rmsQuantN <- sqrt(mean(quantVn^2))

opar <- par(font = 2, font.axis = 2, font.lab = 2,
            las = 1,
            mar = c(5.1, 4.6, 4.1, 2.1),
            lwd = 2, pch = 16)

plot(pvaVn, vipVn,
     col = "red",
     pch = 16,
     xlab = "p-value", ylab = "VIP", xaxs = "i", yaxs = "i")

box(lwd = 2)

curve(qnorm(1 - x / 2) / rmsQuantN, 0, 1, add = TRUE, col = "red", lwd = 3)

abline(h = 1, col = "blue")
abline(v = 0.05, col = "blue")

par(opar)
```
**Figure 6: Relationship between VIP from one-predictive PLS or OPLS models with
standardized variables, and p-values from Pearson correlation test.** The $(p_j,
VIP_j)$ pairs corresponding respectively to the VIP values from OPLS modelling
of the *age* response with the *sacurine* dataset, and the *p*-values from the
Pearson correlation test are shown as red dots. The $y = \Phi^{-1}(1 - x/2) /
z_{rms}$ curve is shown in red (where $\Phi^{-1}$ is the inverse of the
probability density function of the standard normal distribution, and $z_{rms}$
is the quadratic mean of the $z_j$ quantiles from the standard normal
distribution; $z_{rms} = 2.6$ for the *sacurine* dataset and the *age*
response). The vertical (resp. horizontal) blue line corresponds to univariate
(resp. multivariate) thresholds of $p=0.05$ and $VIP=1$, respectively
[@Thevenot2015].

The **VIP** properties above result from:

1. OPLS models of a single response have a single predictive component,

2. in the case of one-predictive component (O)PLS models, the general formula
   for VIPs can be simplified to $VIP_j = \sqrt{m} \times |w_j|$ for each
   feature $j$, were $m$ is the total number of features and **w** is the vector
   of loading weights,

3. in OPLS, **w** remains identical whatever the number of extracted orthogonal
   components,

4. for a single-response model, **w** is proportional to **X'y** (where **'**
denotes the matrix transposition),

5. if **X** and **y** are standardized, **X'y** is the vector of the
   correlations between the features and the response.

@Galindo-Prieto2014 have recently suggested new VIP metrics for OPLS,
**VIP_pred** and **VIP_ortho**, to separately measure the influence of the
features in the modeling of the dispersion correlated to, and orthogonal to the
response, respectively [@Galindo-Prieto2014].

For OPLS(-DA) models, you can therefore get from the model generated with
`opls`:

1. the **predictive VIP vector** (which corresponds to the $VIP_{4,pred}$ metric
measuring the variable importance in prediction) with `getVipVn(model)`,

2. the orthogonal VIP vector which is the $VIP_{4,ortho}$ metric measuring the
variable importance in orthogonal modeling with `getVipVn(model, orthoL =
TRUE)`.  As for the classical **VIP**, we still have the mean of $VIP_{pred}^2$
(and of $VIP_{ortho}^2$) which, each, equals 1.

### (Orthogonal) Partial Least Squares Discriminant Analysis: (O)PLS-DA

#### Two classes

When the **y** response is a factor of 2 levels (character vectors are also
allowed), it is internally transformed into a vector of values $\in \{0,1\}$
encoding the classes. The vector is centered and unit-variance scaled, and the
(O)PLS analysis is performed.

@Brereton2014 have demonstrated that when the sizes of the 2 classes are
**unbalanced**, a **bias** is introduced in the computation of the decision
rule, which penalizes the class with the highest size [@Brereton2014]. In this
case, an external procedure using **resampling** (to balance the classes) and
taking into account the class sizes should be used for optimal results.

#### Multiclass

In the case of **more than 2 levels**, the **y** response is internally
transformed into a matrix (each class is encoded by one column of values $\in
\{0,1\}$). The matrix is centered and unit-variance scaled, and the PLS analysis
is performed.

In this so-called **PLS2** implementation, the proportions of 0 and 1 in the
columns is usually unbalanced (even in the case of balanced size of the classes)
and the bias described previously occurs [@Brereton2014]. The multiclass PLS-DA
results from
[ropls](http://bioconductor.org/packages/release/bioc/html/ropls.html) are
therefore indicative only, and we recommend to set an external procedure where
each column of the matrix is modeled separately (as described above) and the
resulting probabilities are aggregated (see for instance @Bylesjo2006).

## Working on *ExpressionSet* omics objects from bioconductor

The **ExpressionSet** class from the
[Biobase](http://bioconductor.org/packages/release/bioc/html/Biobase.html)
bioconductor package has been developed to conveniently handle preprocessed
omics objects, including the variables x samples matrix of intensities, and data
frames containing the sample and variable metadata [@Huber2015]. The matrix and
the two data frames can be accessed by the `exprs`, `pData` and `fData`
respectively (note that the data matrix is stored in the object with samples in
columns).

The `opls` method can be applied to an **ExpressionSet** object, by using the
object as the `x` argument, and, for (O)PLS(-DA), by indicating as the `y`
argument the name of the sampleMetadata to be used as the response.

In the example below, we will first build a minimal **ExpressionSet** object
from the *sacurine* data set, and we subsequently perform an OPLS-DA.

```{r expressionset_code, warning=FALSE}
library(Biobase)
sacSet <- ExpressionSet(assayData = t(dataMatrix),
                        phenoData = new("AnnotatedDataFrame",
                                        data = sampleMetadata))
opls(sacSet, "gender", orthoI = NA)
```

## Importing/exporting data from/to the Workflow4metabolomics infrastructure

Galaxy is a web-based environment providing powerful graphical user interface
and workflow management functionalities for omics data analysis (@Goecks2010;
@Boekel2015). Wrapping an R code into a Galaxy module is quite straight-forward:
examples can be found on the [toolshed](https://toolshed.g2.bx.psu.edu) central
repository and in the
[RGalaxy](https://www.bioconductor.org/packages/release/bioc/html/RGalaxy.html)
bioconductor package.

[Workflow4metabolomics](http://workflow4metabolomics.org) (W4M) is the online
infrastructure for computational metabolomics based on the Galaxy environment
[@Giacomoni2015]. W4M enables to build, run, save and share workflows
efficiently. In addition, workflows and input/output data (called **histories**)
can be
[referenced](https://galaxy.workflow4metabolomics.org/history/list_published),
thus enabling fully reproducible research. More than 30 modules are currently
available for LC-MS, GC-MS and NMR data preprocessing, statistical analysis, and
annotation, including wrappers of `xcms`, `CAMERA`, `metaMS`, `ropls`, and
`biosigner`, and is open to
[new contributions](http://workflow4metabolomics.org/node/49).

In order to facilitate data import from/to W4M, the `fromW4M` function
(respectively the `toW4M` method) enables import from (respectively export to)
the **W4M 3 tabular file format** (**dataMatrix.tsv**, **sampleMetadata.tsv**,
**variableMetadata.tsv**) into (respectively from) an `ExpressionSet` object, as
shown in the following example which uses the 3 .tsv files stored in the
**extdata** repository of the package to create a *sacSet* ExpressionSet object:

```{r fromW4M}
sacSet <- fromW4M(file.path(path.package("ropls"), "extdata"))
sacSet
```

The generated *sacSet* `ExpressionSet` object can be used with the `opls` method
as described in the previous section.

Conversely, an `ExpressionSet` (with filled **exprs**, **phenoData** and
**featureData** slots) can be exported to the 3 table W4M format:

```{r toW4M, eval = FALSE}
toW4M(sacSet, paste0(getwd(), "/out_"))
```

Before moving to the next session whith another example dataset, we detach
*sacurine* from the search path:

```{r detach}
detach(sacurine)
```

# Other datasets

In addition to the *sacurine* dataset presented above, the package contains
the following datasets to illustrate the functionalities of PCA, PLS and OPLS
(see the examples in the documentation of the opls function):

- **aminoacids** Amino-Acids Dataset. Quantitative structure property
relationship (QSPR) [@Wold2001].

- **cellulose** NIR-Viscosity example data set to illustrate multivariate
calibration using PLS, spectral filtering and OPLS (Multivariate calibration
using spectral data. Simca tutorial. Umetrics, Sweden).

- **cornell** Octane of various blends of gasoline: Twelve mixture
component proportions of the blend are analysed [@Tenenhaus1998].

- **foods** Food consumption patterns accross European countries (FOODS). The
  relative consumption of 20 food items was compiled for 16 countries. The
  values range between 0 and 100 percent and a high value corresponds to a high
  consumption. The dataset contains 3 missing data [@Eriksson2001].

- **linnerud** Three physiological and three exercise variables are measured on
twenty middle-aged men in a fitness club [@Tenenhaus1998].

- **lowarp** A multi response optimization data set (LOWARP) [@Eriksson2001].

- **mark** Marks obtained by french students in mathematics, physics, french and
  english. Toy example to illustrate the potentialities of PCA [@Baccini2010].

# Session info

Here is the output of `sessionInfo()` on the system on which this document was
compiled:

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

# References
