# A Small Note on Box-Cox Transformations

## 2018/03/29

Categories: box-cox Tags: R box-cox

## Preface

In general, data transformations seemed pretty arbitrary to me. I would frequently log or square-root data to make the data follow a distribution. A stickler for procedures, I often wondered, “is there a procedure for finding the best transformation?” Well, luckily there is a procedure, and I have to thank two statisticians with similar sounding names who collaborated with each other - partially because they have similar sounding names.

## What is it?

The Box-Cox procedure performs a data transformation or a regression transformation.

## Why am I using it?

Here, I am only interested in talking about the regression transform, not the data transform.

## What does it look like?

There are several ways to represent the transform[^1], based on the original distribution of the data, but in generalities, it looks like:

$$Y(\lambda) = \Bigg\{\begin{array} 1\frac{Y^{\lambda}-1}{\lambda} & \text{if } \lambda \ne 0 \\ \log(Y) & \text{if } \lambda = 0 \end{array}$$

Here, $Y$ is a random variable, the response variable. Yes, it’s one-sided, and a transform is not performed on the explanatory variables. The assumption is that $Y^k$ behaves with a normal distribution. Therefore, the multivariate density of $Y^k$ is expressed:

$$f_{Y^{\lambda}} = \frac{\exp(-\frac{1}{2}(y^{\lambda} - \mu_y)^T(\Sigma^{-1}) (y^{\lambda} - \mu_y))}{\sqrt{(2\pi)^k | \Sigma |}}$$

The assumption is constant variance, so $\Sigma = \sigma^2 I$. Note $k$ is equal to the index. To get the density of $Y$, the method of transformations (no relation to Box-Cox transformation) is used where the Jacobian is multiplied by the density function of $Y^{\lambda}$

$$f_Y = \frac{\exp(-\frac{1}{2}(y^{\lambda} - \mu_y)^T(\Sigma^{-1}) (y^{\lambda} - \mu_y))}{\sqrt{(2\pi)^k | \Sigma |}} \prod_i^k y^{\lambda-1}$$

Taking the log,

$$\log(f_Y) = \log\Bigg(\frac{\big(\exp(-\frac{1}{2}(y^{\lambda} - \mu_y)^T(\Sigma^{-1}) (y^{\lambda} - \mu_y))}{\sqrt{(2\pi)^k | \Sigma |}} \prod_i^k y^{\lambda-1} \Bigg)$$

$$\log(f_Y) = -\frac{1}{2}(y^{\lambda} - \mu_y)^T(\Sigma^{-1}) (y^{\lambda} - \mu_y) - \frac{k}{2} \log(2\pi \Sigma) + (\lambda-1)\sum_i^k y_i$$

The estimates of $\mu_y, \sigma^2$, and $\lambda$ are done by theoretically taking partials of each. In reality, the estimates can be solved for numerically. Choosing which numerical method is a whole other conversation.

## An Example

The data are housing prices from the AER library. We have one response “price” and eleven predictors. Some of predictors are factors. The box-cox procedure produces: ##  0.1414141

Based on likelihood profile plot above, the estimate for lambda is $0.1414$. Now, I will check visually whether the transformation produced a better model.  A plot of the residuals look better and so does the normal plot.