title: Likert grouping error in R
body: <p>I'm trying to use the grouping argument in the likert function but recive the error:</p>
  
  <pre><code>lik &lt;- likert(df2, grouping = df$CAR)
  
  Error in sum(x) : invalid 'type' (character) of argument
  </code></pre>
  
  <p>Here is my simple code:</p>
  
  <pre><code>library(likert)
  
  df&lt;- fread("C:/R/temp/likert_test.csv", select = 1:6)
  df &lt;- as.data.frame(df)
  
  col_names &lt;- names(df[1:6])
  df[,col_names] &lt;- lapply(df[,col_names] , factor)
  
  str(df)
  'data.frame':   19331 obs. of  6 variables:
   $ CAR                                                     : Factor w/ 34 levels "Alfa Romeo","Audi",..: 4 4 4 4 3 3 3 3 4 4 ...
   $ E1.Overall Satisfaction                                 : Factor w/ 10 levels "1","2","3","4",..: 8 8 8 9 9 7 10 8 7 10 ...
   $ E2.Exterior Styling                                     : Factor w/ 10 levels "1","2","3","4",..: 9 8 8 10 8 4 10 8 7 10 ...
   $ E2.Overall Quality                                      : Factor w/ 10 levels "1","2","3","4",..: 8 8 7 10 10 8 10 8 8 10 ...
   $ E2.Interior Styling                                     : Factor w/ 10 levels "1","2","3","4",..: 9 6 9 10 9 8 9 8 7 10 ...
   $ E2.Quality Of Interior And Materials Used Inside The Car: Factor w/ 10 levels "1","2","3","4",..: 7 6 7 10 10 8 10 7 7 10 ...
  
  
  df2 &lt;- df[,2:5]
  
  lik &lt;- likert(df2, grouping = df$CAR)
  </code></pre>