[R] prediction error in ROCR package when sampled y consists of only one class

[Uwe Ligges]

On 15.04.2011 17:00, Soyeon Kim wrote:
> Dear R users,
>
> Hi. I am using prediction function in ROCR package.
> y consists of two classes 0 and 1.
> However, since I am using cross-validation, a sampled small number of
> y may consist of only one class
>> y
>   [1] 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0

If this is the output for your y, it is not even a classification task, 
since your y is probably numeric rather than a factor.

Hence:
1. make it a factor with the desired number of levels.
2. in this case, the only useful prediction is class "0", hence you can 
easily special case and predict the only available class if only one is 
present.

Best wishes,
Uwe Ligges




> In this case, prediction function gives an error:
> Error in prediction(predic, y) : Number of classes is not equal to 2.
> ROCR currently supports only evaluation of binary classification tasks.
>
> How can I solve this problem?
>
> Thank you,
> Soyeon Kim
>
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