How to set costs matrix for C5.0 Package in R?

Question

I have googled much in the web, but don't find any useful description for the 'costs' parameter for C5.0 function in R. From the C5.0 R manual book, it just says "a matrix of costs associated with the possible errors. The matrix should have C columns and rows where C is the number of class levels". It does not tell me whether the row or the column is the predicated result by the model.

Can anyone help?

Answer 1

Here is a quote from the help page of C5.0 (version 0.1.0-15):

The cost matrix should by CxC, where C is the number of classes. Diagonal elements are ignored. Columns should correspond to the true classes and rows are the predicted classes. For example, if C = 3 with classes Red, Blue and Green (in that order), a value of 5 in the (2,3) element of the matrix would indicate that the cost of predicting a Green sample as Blue is five times the usual value (of one).

Following the example in the help page, this would be a cost matrix:

cost.matrix <- matrix(c(
  NA, 2, 4,
  3, NA, 5,
  7, 1, NA

), 3, 3, byrow=TRUE)

rownames(cost.matrix) <- colnames(cost.matrix) <- c("Red", "Blue", "Green")

cost.matrix

      Red Blue Green
Red    NA    2     4
Blue    3   NA     5
Green   7    1    NA

This would mean the following:

• Predicting a red sample as blue is 3 times the value as the usual value (one)
• Predicting a red sample as green is 7 times the value as the usual
• Predicting a blue sample as red is 2 times the ususal value
• Predicting a blue sample as green is 1 times the ususal value
• Predicting a green sample as red is 4 times the ususal value
• Predicting a green sample as blue is 5 times the usual value