boxcox: Boxcox Power Transformation

Description

boxcox is a generic function used to compute the value(s) of an objective for one or more Box-Cox power transformations, or to compute an optimal power transformation based on a specified objective. The function invokes particular methods which depend on the class of the first argument.

Currently, there is a default method and a method for objects of class "lm".

Usage

boxcox(x, ...)
# S3 method for default
boxcox(x, 
    lambda = {if (optimize) c(-2, 2) else seq(-2, 2, by = 0.5)}, 
    optimize = FALSE, objective.name = "PPCC", 
    eps = .Machine$double.eps, include.x = TRUE, ...)
# S3 method for lm
boxcox(x, 
    lambda = {if (optimize) c(-2, 2) else seq(-2, 2, by = 0.5)}, 
    optimize = FALSE, objective.name = "PPCC", 
    eps = .Machine$double.eps, include.x = TRUE, ...)

Value

When x is an object of class "lm", boxcox returns a list of class "boxcoxLm" containing the results. See the help file for boxcoxLm.object for details.

When x is simply a numeric vector of positive numbers,

boxcox returns a list of class "boxcox" containing the results. See the help file for boxcox.object for details.

Arguments

x: an object of class "lm" for which the response variable is all positive numbers, or else a numeric vector of positive numbers. When x is an object of class "lm", the object must have been created with a call to the function lm that includes the data argument. When x is a numeric vector of positive observations, missing (NA), undefined (NaN), and infinite (-Inf, Inf) values are allowed but will be removed.
lambda: numeric vector of finite values indicating what powers to use for the Box-Cox transformation. When optimize=FALSE, the default value is
lambda=seq(-2, 2, by=0.5). When optimize=TRUE, lambda must be a vector with two values indicating the range over which the optimization will occur and the range of these two values must include 1. In this case, the default value is lambda=c(-2, 2).
optimize: logical scalar indicating whether to simply evalute the objective function at the given values of lambda (optimize=FALSE; the default), or to compute the optimal power transformation within the bounds specified by lambda (optimize=TRUE).
objective.name: character string indicating what objective to use. The possible values are "PPCC" (probability plot correlation coefficient; the default), "Shapiro-Wilk" (the Shapiro-Wilk goodness-of-fit statistic), and "Log-Likelihood" (the log-likelihood function).
eps: finite, positive numeric scalar. When the absolute value of lambda is less than eps, lambda is assumed to be 0 for the Box-Cox transformation. The default value is eps=.Machine$double.eps.
include.x: logical scalar indicating whether to include the finite, non-missing values of the argument x with the returned object. The default value is include.x=TRUE.
...: optional arguments for possible future methods. Currently not used.

Author

Steven P. Millard (EnvStats@ProbStatInfo.com)

Details

Two common assumptions for several standard parametric hypothesis tests are:

The observations all come from a normal distribution.
The observations all come from distributions with the same variance.

For example, the standard one-sample t-test assumes all the observations come from the same normal distribution, and the standard two-sample t-test assumes that all the observations come from a normal distribution with the same variance, although the mean may differ between the two groups.

When the original data do not satisfy the above assumptions, data transformations are often used to attempt to satisfy these assumptions. The rest of this section is divided into two parts: one that discusses Box-Cox transformations in the context of the original observations, and one that discusses Box-Cox transformations in the context of linear models.

Box-Cox Transformations Based on the Original Observations
Box and Cox (1964) presented a formalized method for deciding on a data transformation. Given a random variable $X$ from some distribution with only positive values, the Box-Cox family of power transformations is defined as:

$Y$	=	$\frac{X^{λ} - 1}{λ}$	$λ \neq 0$

		$l o g (X)$	$λ = 0 (1)$

where $Y$ is assumed to come from a normal distribution. This transformation is continuous in $λ$ . Note that this transformation also preserves ordering. See the help file for boxcoxTransform for more information on data transformations.

Let $\underset{―}{x} = x_{1}, x_{2}, \dots, x_{n}$ denote a random sample of $n$ observations from some distribution and assume that there exists some value of $λ$ such that the transformed observations

$y_{i}$	=	$\frac{x_{i}^{λ} - 1}{λ}$	$λ \neq 0$

		$l o g (x_{i})$	$λ = 0 (2)$

( $i = 1, 2, \dots, n$ ) form a random sample from a normal distribution.

Box and Cox (1964) proposed choosing the appropriate value of $λ$ based on maximizing the likelihood function. Alternatively, an appropriate value of $λ$ can be chosen based on another objective, such as maximizing the probability plot correlation coefficient or the Shapiro-Wilk goodness-of-fit statistic.

In the case when optimize=TRUE, the function boxcox calls the R function nlminb to minimize the negative value of the objective (i.e., maximize the objective) over the range of possible values of $λ$ specified in the argument lambda. The starting value for the optimization is always $λ = 1$ (i.e., no transformation).

The rest of this sub-section explains how the objective is computed for the various options for objective.name.

Objective Based on Probability Plot Correlation Coefficient (objective.name="PPCC")
When objective.name="PPCC", the objective is computed as the value of the normal probability plot correlation coefficient based on the transformed data (see the description of the Probability Plot Correlation Coefficient (PPCC) goodness-of-fit test in the help file for gofTest). That is, the objective is the correlation coefficient for the normal quantile-quantile plot for the transformed data. Large values of the PPCC tend to indicate a good fit to a normal distribution.

Objective Based on Shapiro-Wilk Goodness-of-Fit Statistic (objective.name="Shapiro-Wilk")
When objective.name="Shapiro-Wilk", the objective is computed as the value of the Shapiro-Wilk goodness-of-fit statistic based on the transformed data (see the description of the Shapiro-Wilk test in the help file for gofTest). Large values of the Shapiro-Wilk statistic tend to indicate a good fit to a normal distribution.

Objective Based on Log-Likelihood Function (objective.name="Log-Likelihood")
When objective.name="Log-Likelihood", the objective is computed as the value of the log-likelihood function. Assuming the transformed observations in Equation (2) above come from a normal distribution with mean $μ$ and standard deviation $σ$ , we can use the change of variable formula to write the log-likelihood function as: $l o g [L (λ, μ, σ)] = \frac{- n}{2} l o g (2 π) - \frac{n}{2} l o g (σ^{2}) - \frac{1}{2 σ^{2}} \sum_{i = 1}^{n} (y_{i} - μ)^{2} + (λ - 1) \sum_{i = 1}^{n} l o g (x_{i}) (3)$ where $y_{i}$ is defined in Equation (2) above (Box and Cox, 1964). For a fixed value of $λ$ , the log-likelihood function is maximized by replacing $μ$ and $σ$ with their maximum likelihood estimators: $\hat{μ} = \frac{1}{n} \sum_{i = 1}^{n} y_{i} (4)$ $\hat{σ} = [\frac{1}{n} \sum_{i = 1}^{n} (y_{i} - \bar{y})^{2}]^{1 / 2} (5)$ Thus, when optimize=TRUE, Equation (3) is maximized by iteratively solving for $λ$ using the values for $μ$ and $σ$ given in Equations (4) and (5). When optimize=FALSE, the value of the objective is computed by using Equation (3), using the values of $λ$ specified in the argument lambda, and using the values for $μ$ and $σ$ given in Equations (4) and (5).

Box-Cox Transformation for Linear Models
In the case of a standard linear regression model with $n$ observations and $p$ predictors: $Y_{i} = β_{0} + β_{1} X_{i 1} + \dots + β_{p} X_{i p} + ϵ_{i}, i = 1, 2, \dots, n (6)$ the standard assumptions are:

The error terms $ϵ_{i}$ come from a normal distribution with mean 0.
The variance is the same for all of the error terms and does not depend on the predictor variables.

Assuming $Y$ is a random variable from some distribution that may depend on the predictor variables and $Y$ takes on only positive values, the Box-Cox family of power transformations is defined as:

$Y^{*}$	=	$\frac{Y^{λ} - 1}{λ}$	$λ \neq 0$

		$l o g (Y)$	$λ = 0 (7)$

where $Y^{*}$ becomes the new response variable and the errors are now assumed to come from a normal distribution with a mean of 0 and a constant variance.

In this case, the objective is computed as described above, but it is based on the residuals from the fitted linear model in which the response variable is now $Y^{*}$ instead of $Y$ .

References

Berthouex, P.M., and L.C. Brown. (2002). Statistics for Environmental Engineers, Second Edition. Lewis Publishers, Boca Raton, FL.

Box, G.E.P., and D.R. Cox. (1964). An Analysis of Transformations (with Discussion). Journal of the Royal Statistical Society, Series B 26(2), 211--252.

Draper, N., and H. Smith. (1998). Applied Regression Analysis. Third Edition. John Wiley and Sons, New York, pp.47-53.

Gilbert, R.O. (1987). Statistical Methods for Environmental Pollution Monitoring. Van Nostrand Reinhold, NY.

Helsel, D.R., and R.M. Hirsch. (1992). Statistical Methods in Water Resources Research. Elsevier, New York, NY.

Hinkley, D.V., and G. Runger. (1984). The Analysis of Transformed Data (with Discussion). Journal of the American Statistical Association 79, 302--320.

Hoaglin, D.C., F.M. Mosteller, and J.W. Tukey, eds. (1983). Understanding Robust and Exploratory Data Analysis. John Wiley and Sons, New York, Chapter 4.

Hoaglin, D.C. (1988). Transformations in Everyday Experience. Chance 1, 40--45.

Johnson, N. L., S. Kotz, and A.W. Kemp. (1992). Univariate Discrete Distributions, Second Edition. John Wiley and Sons, New York, p.163.

Johnson, R.A., and D.W. Wichern. (2007). Applied Multivariate Statistical Analysis, Sixth Edition. Pearson Prentice Hall, Upper Saddle River, NJ, pp.192--195.

Shumway, R.H., A.S. Azari, and P. Johnson. (1989). Estimating Mean Concentrations Under Transformations for Environmental Data With Detection Limits. Technometrics 31(3), 347--356.

Stoline, M.R. (1991). An Examination of the Lognormal and Box and Cox Family of Transformations in Fitting Environmental Data. Environmetrics 2(1), 85--106.

van Belle, G., L.D. Fisher, Heagerty, P.J., and Lumley, T. (2004). Biostatistics: A Methodology for the Health Sciences, 2nd Edition. John Wiley & Sons, New York.

Zar, J.H. (2010). Biostatistical Analysis. Fifth Edition. Prentice-Hall, Upper Saddle River, NJ, Chapter 13.

Examples

Run this code

  # Generate 30 observations from a lognormal distribution with 
  # mean=10 and cv=2.  Look at some values of various objectives 
  # for various transformations.  Note that for both the PPCC and 
  # the Log-Likelihood objective, the optimal value of lambda is 
  # about 0, indicating that a log transformation is appropriate.  
  # (Note: the call to set.seed simply allows you to reproduce this example.)

  set.seed(250) 
  x <- rlnormAlt(30, mean = 10, cv = 2) 

  dev.new()
  hist(x, col = "cyan")

  # Using the PPCC objective:
  #--------------------------

  boxcox(x) 
  #Results of Box-Cox Transformation
  #---------------------------------
  #
  #Objective Name:                  PPCC
  #
  #Data:                            x
  #
  #Sample Size:                     30
  #
  # lambda      PPCC
  #   -2.0 0.5423739
  #   -1.5 0.6402782
  #   -1.0 0.7818160
  #   -0.5 0.9272219
  #    0.0 0.9921702
  #    0.5 0.9581178
  #    1.0 0.8749611
  #    1.5 0.7827009
  #    2.0 0.7004547

  boxcox(x, optimize = TRUE)
  #Results of Box-Cox Transformation
  #---------------------------------
  #
  #Objective Name:                  PPCC
  #
  #Data:                            x
  #
  #Sample Size:                     30
  #
  #Bounds for Optimization:         lower = -2
  #                                 upper =  2
  #
  #Optimal Value:                   lambda = 0.04530789
  #
  #Value of Objective:              PPCC = 0.9925919


  # Using the Log-Likelihodd objective
  #-----------------------------------

  boxcox(x, objective.name = "Log-Likelihood") 
  #Results of Box-Cox Transformation
  #---------------------------------
  #
  #Objective Name:                  Log-Likelihood
  #
  #Data:                            x
  #
  #Sample Size:                     30
  #
  # lambda Log-Likelihood
  #   -2.0     -154.94255
  #   -1.5     -128.59988
  #   -1.0     -106.23882
  #   -0.5      -90.84800
  #    0.0      -85.10204
  #    0.5      -88.69825
  #    1.0      -99.42630
  #    1.5     -115.23701
  #    2.0     -134.54125

  boxcox(x, objective.name = "Log-Likelihood", optimize = TRUE) 
  #Results of Box-Cox Transformation
  #---------------------------------
  #
  #Objective Name:                  Log-Likelihood
  #
  #Data:                            x
  #
  #Sample Size:                     30
  #
  #Bounds for Optimization:         lower = -2
  #                                 upper =  2
  #
  #Optimal Value:                   lambda = 0.0405156
  #
  #Value of Objective:              Log-Likelihood = -85.07123

  #----------

  # Plot the results based on the PPCC objective
  #---------------------------------------------
  boxcox.list <- boxcox(x)
  dev.new()
  plot(boxcox.list)

  #Look at QQ-Plots for the candidate values of lambda
  #---------------------------------------------------
  plot(boxcox.list, plot.type = "Q-Q Plots", same.window = FALSE) 

  #==========

  # The data frame Environmental.df contains daily measurements of 
  # ozone concentration, wind speed, temperature, and solar radiation
  # in New York City for 153 consecutive days between May 1 and 
  # September 30, 1973.  In this example, we'll plot ozone vs. 
  # temperature and look at the Q-Q plot of the residuals.  Then 
  # we'll look at possible Box-Cox transformations.  The "optimal" one 
  # based on the PPCC looks close to a log-transformation 
  # (i.e., lambda=0).  The power that produces the largest PPCC is 
  # about 0.2, so a cube root (lambda=1/3) transformation might work too.

  head(Environmental.df)
  #           ozone radiation temperature wind
  #05/01/1973    41       190          67  7.4
  #05/02/1973    36       118          72  8.0
  #05/03/1973    12       149          74 12.6
  #05/04/1973    18       313          62 11.5
  #05/05/1973    NA        NA          56 14.3
  #05/06/1973    28        NA          66 14.9

  tail(Environmental.df)
  #           ozone radiation temperature wind
  #09/25/1973    14        20          63 16.6
  #09/26/1973    30       193          70  6.9
  #09/27/1973    NA       145          77 13.2
  #09/28/1973    14       191          75 14.3
  #09/29/1973    18       131          76  8.0
  #09/30/1973    20       223          68 11.5


  # Fit the model with the raw Ozone data
  #--------------------------------------
  ozone.fit <- lm(ozone ~ temperature, data = Environmental.df) 

  # Plot Ozone vs. Temperature, with fitted line 
  #---------------------------------------------
  dev.new()
  with(Environmental.df, 
    plot(temperature, ozone, xlab = "Temperature (degrees F)",
      ylab = "Ozone (ppb)", main = "Ozone vs. Temperature"))
  abline(ozone.fit) 

  # Look at the Q-Q Plot for the residuals 
  #---------------------------------------
  dev.new()
  qqPlot(ozone.fit$residuals, add.line = TRUE) 

  # Look at Box-Cox transformations of Ozone 
  #-----------------------------------------
  boxcox.list <- boxcox(ozone.fit) 
  boxcox.list 
  #Results of Box-Cox Transformation
  #---------------------------------
  #
  #Objective Name:                  PPCC
  #
  #Linear Model:                    ozone.fit
  #
  #Sample Size:                     116
  #
  # lambda      PPCC
  #   -2.0 0.4286781
  #   -1.5 0.4673544
  #   -1.0 0.5896132
  #   -0.5 0.8301458
  #    0.0 0.9871519
  #    0.5 0.9819825
  #    1.0 0.9408694
  #    1.5 0.8840770
  #    2.0 0.8213675

  # Plot PPCC vs. lambda based on Q-Q plots of residuals 
  #-----------------------------------------------------
  dev.new()
  plot(boxcox.list) 

  # Look at Q-Q plots of residuals for the various transformation 
  #--------------------------------------------------------------
  plot(boxcox.list, plot.type = "Q-Q Plots", same.window = FALSE)

  # Compute the "optimal" transformation
  #-------------------------------------
  boxcox(ozone.fit, optimize = TRUE)
  #Results of Box-Cox Transformation
  #---------------------------------
  #
  #Objective Name:                  PPCC
  #
  #Linear Model:                    ozone.fit
  #
  #Sample Size:                     116
  #
  #Bounds for Optimization:         lower = -2
  #                                 upper =  2
  #
  #Optimal Value:                   lambda = 0.2004305
  #
  #Value of Objective:              PPCC = 0.9940222

  #==========

  # Clean up
  #---------
  rm(x, boxcox.list, ozone.fit)
  graphics.off()

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