Can a non-certified real-estate appraiser work on his/her own?

I'm pretty interested in real-estate appraising and I'm having a hard time finding out whether a non-certified real estate appraiser can work on his or her own. I know there are various levels of appraisers. It goes Trainee,Residential appraiser,Certified residential, and then Certified General. Any help would be much appreciated. Thanks