Another in my series of cheat sheets on the R programming language. This one covers factors. [The link is to a directory in Google documents. There you will find my other cheat sheets, which I have also updated].
When I first started learning R, I had more problems with factors than just about anything else. Behind the scenes, R has a default modality of converting some vectors to factors when they get put into new data frames. It tripped me up quite a few times.
As always, let me know if there are any errors or obvious omissions.