I'm using liblinear to classify documents. The input are arrays of tf-idf scores. Everything is working well, but now I'm trying to determine optimal parameters. I set up a loop of cross validation operations with cost incrementing from loop to loop. However, even over hundreds of iterations I see no effect from changing C. The results are all within 2% of each other, and vary so little that even just running a loop with a single C value can produce the same spread of results. What am I doing wrong or not understanding?
Note: I am using the Ruby binding for liblinear, not the command line tools or direct C code. That said the Ruby binding is a very, very small wrapper around the C library itself and works and acts exactly the same.