title: using the boxTidwell function in the CAR package and getting a bizarre error
body: <p>I am trying to use the boxTidwell function in the CAR package in r to run a number of tests on continuous data. My data looks something like this:</p>
  
  <pre><code>Gender Age    X1      X2   Outcome
    M    20.1   1.23   4.43     1
    F    19.5   2.33   3.21     0
    M    18.0   1.33   7.55     1
    M    17.2   3.22   6.44     0
    M    12.5   4.15   8.99     1
    F    14.2   5.15  10.22     0
    F    13.9   6.12  12.34     1 
    F     9.4   7.12   3.21     1
  </code></pre>
  
  <p>When I use boxTidwell on the dataframe, I get an error</p>
  
  <pre><code>library(car)    
  gender&lt;-c("M","F","M","M","M","F","F","F")
      age&lt;-c(20.1, 19.5, 18.0, 17.2, 12.5, 14.2, 13.9, 9.4)
      X1&lt;-c(1.23,2.33,1.33,3.22,4.15,5.15,6.12,7.12)
      X2&lt;-c(4.43,3.21,7.55,6.44,8.99,10.22,12.34,3.21)
      outcome&lt;-c(1,0,1,0,1,0,1,1)
      df&lt;-cbind(gender,age,X1,X2,outcome)
      as.data.frame(df)
      boxTidwell(outcome~age+X1+X2, ~gender, data=df)
  </code></pre>
  
  <blockquote>
    <p>Error in boxTidwell.default(y, X1, X2, max.iter = max.iter, tol = tol,  : 
      the variables to be transformed must have only positive values
    In addition: Warning message:
    In model.response(mf, "numeric") :
      using type = "numeric" with a factor response will be ignored</p>
  </blockquote>
  
  <p>I am not sure what the problem is, I assume it is because I am using a binary outcome. Any suggestion would be much appreciated</p>