---
title: "Principal Component Analysis"
author: "Philippe Grosjean"
date: "`r Sys.Date()`"
output:
  rmarkdown::html_vignette:
    toc: true
    toc_depth: 3
    fig_caption: yes
vignette: >
  %\VignetteIndexEntry{Principal Component Analysis}
  %\VignetteEngine{knitr::rmarkdown}
  %\VignetteEncoding{UTF-8}
---

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

Principal Component Analysis, or PCA is a widely used method to explore linear associations among variables of large datasets. There is, unfortunately, no consistent implementation of this technique in R, which is even more a problem because the numerous additional R packages that provide enhanced versions of PCA, or additional tools, have no consistent interfaces. In the **stats** package, there are two implementations called `princomp()` and `prcomp()` that both create S3 objects of the same name. There are a few methods available, like `print()`, `summary()`, `plot()`, `predict()` or `biplot()`. The whole set is rather deceptive and produces less interesting plots than other (more specialized) software can do. For instance, there is nothing to plot the so-called "graph of the variables" in the French terminology and you have to program it yourself.

Of course, there are several specialized R packages available that provide more powerful and/or more extended implementations, among others: **ade4**, **FactoMineR** and **vegan**. Each of these packages has a totally different approach: **ade4** creates a `c('pca', 'dudi')` S3 object and proposes nice graphs but has an interface that is completely inconsistent with usual R analyses (no optional formula interface, exotic names of arguments, non-standard handling of missing data, etc.). Object orientation and name of objects are obscure and do not facilitate first use of the PCA in **ade4**. A PCA is done, indeed, using the `dudi.pca()` function (or possibly, `nipals()`, but that creates a different `nipals` object). The same remarks can be made about the interface of functions in **FactoMineR**: they use strange arguments and do not respect the general organization of analyses in R (an object constructs the analysis, possibly defined using a formula; methods summarize or plot the results piece by piece). At least, name of function and object related to PCA are clear in **FactoMineR**: `PCA()`. There is also a non conventional handling of missing observations. But the function is powerful and allows for several investigations around the PCA. In **vegan**, there is no PCA function, but a redundancy analysis `rda()`, which reduces to a classical PCA when arguments `X =` and `Y =` are missing. It creates a `c('cca', 'rda')` S3 object which is not optimized at all for holding pure PCA data (many unnecessary items in it for a PCA). Finally, **labdsv** uses the default `prcomp()`, but it wraps it into a `pca` S3 object, in order to define additional plotting methods that are consistent with the other analyses and objects in that package. Note that both `pca` S3 objects in **ade4** and **labdsv** are completely different, and you are likely to get very bad results in case you load both packages and mix their respective methods!

So, given that chaotic set of PCA functions in R, would it be possible to design an object with minimal code that reuses code in the stats package (`princomp()` and `prcomp()`), provides a couple of additional methods to make decent variables and individuals plots (possibly with ellipses or convex hulls for subgroups) in a way that a whole analysis would be easy to perform and to read in R code? We have tried to do so in the present **SciViews** package.

It would have been nice to name our object `pca`. But that `pca` S3 object could not be compatible with **ade4**’s `pca` object. So, we decided to give it a different name: `pcomp`.

##  The `pcomp` object

The `pcomp` S3 object, inherits from `pca` and `princomp`. It returns a list with components:

- `loadings`: (also required for **labdsv**’s `pca` object). This is `$rotation` in `prcomp`, and a `loadings` object in `princomp`,

- `scores`: (also required for **labdsv**’s `pca` object). Note for scores in `princomp`, components are `Comp.1`, `Comp.2`, etc., in `prcomp`, they are named `PC1`, `PC2`, ..., as well as in `pca`). So, we keep `PC1`, `PC2`, ... This is `$x` in `prcomp`. For `princomp()`, the argument `scores = TRUE` (by default) must be used to get these.

- `sdev`: (also required for **labdsv*’s `pca` object). `princomp()` uses names (to rename into `PC1`, `PC2`, ...), while `prcomp()` does not,

- `totdev`: the total deviance, as required to be compliant with **labdsv**’s `pca` object.

- `n.obs`: the number of observations,

- `center`: (use 0 for all, if not centered),

- `scale`: (use 1 for all, if not scaled),

- `method`: currently only either `svd` (and the computation is the same as `prcomp()`), or `eigen` (and the computation is the same as `princomp()`),

- `call`: the matched call,

- `na.action`: if relevant.
