symbox: Boxplots for transformations to symmetry In car: Companion to Applied Regression

 symbox R Documentation

Boxplots for transformations to symmetry

Description

`symbox` first transforms `x` to each of a series of selected powers, with each transformation standardized to mean 0 and standard deviation 1. The results are then displayed side-by-side in boxplots, permiting a visual assessment of which power makes the distribution reasonably symmetric. For the `"lm"` method, the response variable in the model is successively transformed.

Usage

```symbox(x, ...)
## S3 method for class 'formula'
symbox(formula, data=NULL, subset, na.action=NULL, ylab,  ...)
## Default S3 method:
symbox(x, powers = c(-1, -0.5, 0, 0.5, 1), start,
trans=bcPower, xlab="Powers", ylab, ...)
## S3 method for class 'lm'
symbox(x, powers = c(-1, -0.5, 0, 0.5, 1), start, trans=bcPower,
xlab, ylab="Studentized residuals", ...)
```

Arguments

 `x` a numeric vector. `formula` a one-sided formula specifying a single numeric variable. `data, subset, na.action` as for statistical modeling functions (see, e.g., `lm`). `xlab, ylab` axis labels; if `ylab` is missing, a label will be supplied. For the `"lm"` method, if `xlab` is missing, a label will also be supplied. `powers` a vector of selected powers to which `x` is to be raised. For meaningful comparison of powers, `1` should be included in the vector of powers. `start` a constant to be added to `x`. If `start` is missing and `trans` is `bcPower` (the default) or `bcnPower`, then a start will be automatically generated if there are zero or negative values in `x`, and a warning will be printed; the auto-generated `start` is the absolute value of the minimum `x` plus 1 percent of the range of `x`. `trans` a transformation function whose first argument is a numeric vector and whose second argument is a transformation parameter, given by the `powers` argument; the default is `bcPower`, and another possibility is `yjPower`. `bcnPower` may also be used, in which case the `gamma` parameter is set to the value of `start`. `...` arguments to be passed down.

Value

as returned by `boxplot`.

Author(s)

Gregor Gorjanc, John Fox jfox@mcmaster.ca.

References

Fox, J. and Weisberg, S. (2019) An R Companion to Applied Regression, Third Edition. Sage.

`boxplot`, `boxcox`, `bcPower`, `yjPower`
```symbox(~ income, data=Prestige)