Dummy variables (experimental)

Largely a wrapper for model.matrix that accepts a factor, f, and returns a dummy matrix with nlevels(f)-1 columns (the first column is dropped by default). Useful whenever one wishes to avoid the behaviour of model.matrix of always returning an nlevels(f)-column matrix, either by the addition of an intercept column, or by keeping one column for all levels.

dummy(f, levelsToKeep)

Arguments

f

An object coercible to factor.

levelsToKeep

An optional character vector giving the subset of levels(f) to be converted to dummy variables.

Value

A model.matrix with dummy variables as columns.

Examples

data(Orthodont,package="nlme")
lmer(distance ~ age + (age|Subject) +
     (0+dummy(Sex, "Female")|Subject), data = Orthodont)
#> Linear mixed model fit by REML ['lmerMod']
#> Formula: distance ~ age + (age | Subject) + (0 + dummy(Sex, "Female") |  
#>     Subject)
#>    Data: Orthodont
#> REML criterion at convergence: 442.5444
#> Random effects:
#>  Groups    Name                 Std.Dev. Corr 
#>  Subject   (Intercept)          2.3439        
#>            age                  0.2264   -0.66
#>  Subject.1 dummy(Sex, "Female") 1.0521        
#>  Residual                       1.3100        
#> Number of obs: 108, groups:  Subject, 27
#> Fixed Effects:
#> (Intercept)          age  
#>     16.8643       0.6602