---
title: "Gram-Schmidt Orthogonalization and Regression"
author: "Michael Friendly"
date: "`r Sys.Date()`"
output: rmarkdown::html_vignette
vignette: >
%\VignetteIndexEntry{Gram-Schmidt Orthogonalization and Regression}
%\VignetteEngine{knitr::rmarkdown}
%\VignetteEncoding{UTF-8}
---
```{r nomessages, echo = FALSE}
knitr::opts_chunk$set(
warning = FALSE,
message = FALSE
)
options(digits=4)
par(mar=c(5,4,1,1)+.1)
```
This vignette illustrates the process of transforming a set of variables to a new set of uncorrelated (orthogonal)
variables. It carries out the Gram-Schmidt process **directly** by successively projecting each successive variable
on the previous ones and subtracting (taking residuals). This is equivalent by replacing each successive variable by
its residuals from a least squares regression on the previous variables.
When this method is used on the predictors in a regression problem,
the resulting orthogonal variables have exactly the same `anova()` summary (based on "Type I", sequential sums of squares)
as do original variables.
## Setup
We use the `class` data set, but convert the character factor `sex` to a dummy (0/1) variable `male`.
```{r class1}
library(matlib)
data(class)
class$male <- as.numeric(class$sex=="M")
```
For later use in regression, we create a variable `IQ` as a response variable
```{r class2}
class <- transform(class,
IQ = round(20 + height + 3*age -.1*weight -3*male + 10*rnorm(nrow(class))))
head(class)
```
Reorder the predictors we want, forming a numeric matrix, `X`.
```{r class3}
X <- as.matrix(class[,c(3,4,2,5)])
head(X)
```
## Orthogonalization by projections
The Gram-Schmidt process treats the variables in a given order, according to the columns in `X`.
We start with a new matrix `Z` consisting of `X[,1]`.
Then, find a new variable `Z[,2]` orthogonal to `Z[,1]` by subtracting the projection of `X[,2]` on `Z[,1]`.
```{r}
Z <- cbind(X[,1], 0, 0, 0)
Z[,2] <- X[,2] - Proj(X[,2], Z[,1])
crossprod(Z[,1], Z[,2]) # verify orthogonality
```
Continue in the same way, subtracting the projections of `X[,3]` on the previous columns, and so forth
```{r}
Z[,3] <- X[,3] - Proj(X[,3], Z[,1]) - Proj(X[,3], Z[,2])
Z[,4] <- X[,4] - Proj(X[,4], Z[,1]) - Proj(X[,4], Z[,2]) - Proj(X[,4], Z[,3])
```
Note that if any column of `X` is a linear combination of the previous columns, the corresponding
column of `Z` will be all zeros.
These computations are similar to the following set of linear regressions:
```{r usinglm}
z2 <- residuals(lm(X[,2] ~ X[,1]), type="response")
z3 <- residuals(lm(X[,3] ~ X[,1:2]), type="response")
z4 <- residuals(lm(X[,4] ~ X[,1:3]), type="response")
```
The columns of `Z` are now orthogonal, but not of unit length,
```{r ortho1}
zapsmall(crossprod(Z)) # check orthogonality
```
We make standardize column to unit length, giving `Z` as an **orthonormal** matrix, such that $Z' Z = I$.
```{r ortho2}
Z <- Z %*% diag(1 / len(Z)) # make each column unit length
zapsmall(crossprod(Z)) # check orthonormal
colnames(Z) <- colnames(X)
```
### Relationship to QR factorization
The QR method uses essentially the same process, factoring the matrix $\mathbf{X}$ as $\mathbf{X = Q R}$,
where $\mathbf{Q}$ is the orthonormal matrix corresponding to `Z` and $\mathbf{R}$ is an upper triangular matrix.
However, the signs of the columns of $\mathbf{Q}$ are arbitrary, and `QR()` returns `QR(X)$Q` with
signs reversed, compared to `Z`.
```{r QR}
# same result as QR(X)$Q, but with signs reversed
head(Z, 5)
head(-QR(X)$Q, 5)
all.equal( unname(Z), -QR(X)$Q )
```
## Regression with X and Z
We carry out two regressions of `IQ` on the variables in `X` and in `Z`. These are equivalent, in the sense that
- The $R^2$ and MSE are the same in both models
- Residuals are the same
- The Type I tests given by `anova()` are the same.
```{r class2IQ}
class2 <- data.frame(Z, IQ=class$IQ)
```
Regression of IQ on the original variables in `X`
```{r mod1, R.options=list(digits=5)}
mod1 <- lm(IQ ~ height + weight + age + male, data=class)
anova(mod1)
```
Regression of IQ on the orthogonalized variables in `Z`
```{r mod2, R.options=list(digits=5)}
mod2 <- lm(IQ ~ height + weight + age + male, data=class2)
anova(mod2)
```
This illustrates that `anova()` tests for linear models are *sequential* tests. They test hypotheses about the
extra contribution of each variable over and above all previous ones, in a given order. These usually do not make
substantive sense, except in testing ordered ("hierarchical") models.