6. The distributed data annotation server system of claim 5 , wherein the source data metadata includes a measure of the difficulty of describing the source data.
6. The distributed data annotation server system of claim 5 , wherein the source data metadata includes a measure of the difficulty of describing the source data. 7. The distributed data annotation server system of claim 6 , wherein the source data metadata further comprises source data characteristics selected from the group consisting of annotations applied to the piece of source data, features of the source data, and annotators who have previously annotated the piece of source data.