# dummy.coef: Extract Coefficients in Original Coding

## Description

This extracts coefficients in terms of the original levels of the coefficients rather than the coded variables.

## Usage

 ```1 2 3 4 5 6 7``` ```dummy.coef(object, ...) ## S3 method for class 'lm' dummy.coef(object, use.na = FALSE, ...) ## S3 method for class 'aovlist' dummy.coef(object, use.na = FALSE, ...) ```

## Arguments

 `object` a linear model fit. `use.na` logical flag for coefficients in a singular model. If `use.na` is true, undetermined coefficients will be missing; if false they will get one possible value. `...` arguments passed to or from other methods.

## Details

A fitted linear model has coefficients for the contrasts of the factor terms, usually one less in number than the number of levels. This function re-expresses the coefficients in the original coding; as the coefficients will have been fitted in the reduced basis, any implied constraints (e.g., zero sum for `contr.helmert` or `contr.sum`) will be respected. There will be little point in using `dummy.coef` for `contr.treatment` contrasts, as the missing coefficients are by definition zero.

The method used has some limitations, and will give incomplete results for terms such as `poly(x, 2)`. However, it is adequate for its main purpose, `aov` models.

## Value

A list giving for each term the values of the coefficients. For a multistratum `aov` model, such a list for each stratum.

## Warning

This function is intended for human inspection of the output: it should not be used for calculations. Use coded variables for all calculations.

The results differ from S for singular values, where S can be incorrect.

`aov`, `model.tables`
 ```1 2 3 4 5 6 7``` ```options(contrasts = c("contr.helmert", "contr.poly")) ## From Venables and Ripley (2002) p.165. npk.aov <- aov(yield ~ block + N*P*K, npk) dummy.coef(npk.aov) npk.aovE <- aov(yield ~ N*P*K + Error(block), npk) dummy.coef(npk.aovE) ```