biased random guess classification

Question

I try to get used to some classification methods in R (kNN, Decision Trees, SVM) and I am just wondering: Is there a way to do a biased random guess classification to see the real performance of the classificator?

Update: Example: There are two classes, but its a imbalanced data set. Class 1 makes 70 %, class 2 30 %. Therefore, its not a big deal do "guess" 70 % correct by classifying each data record as class 1. So I want to show the following: Classificator: 90 % identified as TP random guess (biased, for the known distribution): 73 %

The random guess should just identify the data records by guessing. If the distribution would be balanced, it would show approx. a 50/50 result. With the known distribution it would show a approx. 70/30 result. Hopefully this clarifies the question a little bit...

Thanks!

Answer 1

In Matlab you'd do this for a single observation (assuming a binary classification):

p = 0.7;
y = rand() > p;

or if your input set of binary labels is a vector Y then you can do:

p = mean(Y);
y = rand(size(Y)) <= p;

I imagine the R code would be nearly identical.